# Tuesday’s Child

Following on from the teasers in the probability paradoxes post, here is a closer look at “Tuesday’s child”. While it may not strictly be a paradox, it still has the rich potential for generating controversy. In fact, I don’t agree with what could be called the “classical” analysis of the problem. Here I will look at this classical approach and save my own interpretation for a later post.

A warning: this will be the most mathematical post on the blog to date, so it is not for the faint-hearted!

All of these probability paradoxes hinge on the notation of conditional probability. Conditional probability is the probability of one event given that another event has occurred. As a simple example, imagine roll a dice and A denotes “rolling a six” and B denotes “rolling an even number”. Then the probability of rolling a six is 1/6, but the probability of rolling a 6 given that I roll an even number is 1/3.

Now, before getting onto Tuesday’s child, I will go back to the simpler paradox, which I introduced in the Martin Gardner post. Note that throughout this post I will assume that girls and boys are equally likely and I will ignore identical twins (no offence to identical twins, of course!). I’ll quote from Gardner’s “Mathematical Puzzles and Diversions”:

Mr Smith says, ‘I have two children and at least one of them is a boy.’ What is the probability that the other child is a boy? One is tempted to say 1/2 until he lists the three possible combinations of equally probable possibilities – BB, BG, GB. Only one is BB, hence the probability is 1/3. Had Smith said that his oldest (or tallest, heaviest, etc.) child is a boy, then the situation is entirely different. Now the combinations are restricted to BB and BG, and the probability that the other child is male jumps to 1/2.

Without going into my reasons in this post, I don’t agree with Gardner’s solution to the problem as he posed it. But, with a little tweak, I would agree. If instead you ask Mr Smith whether he has at least one boy among his two children and he says ‘yes’ (as opposed to having him volunteer the details), then the probability that he has two boys is 1/3.

I’ll tweak the Tuesday’s child problem in the same way for now. Imagine the problem is now as follows:

Mr Smith has two children. You ask whether he has at least one boy born on a Tuesday. He says ‘yes’. A lucky guess perhaps, but now you wonder what the chances are that Mr Smith’s other child is also a boy.

At this point, we could enumerate all the possible combinations of children and weekdays of birth. All up there are $2\times 7\times 2\times 7 = 196$ possibilities. Looking through that list, we would then scratch all of those that do not have at least one boy born on a Tuesday. In the list that remains, look at the proportion made up by families with two boys. Try it and you’ll find your revised list has 27 combinations in it and 13 of them have two boys, so the probability we are after is 13/27.

For me, that looks a bit much like hard work, so instead I would call on a bit more of the mathematics of conditional probability. Feel free to stop reading now if you don’t have the stomach for even more mathematics!

Mathematically, conditional probability is defined as follows:

$P(A | B) = \frac{P(A \cap B)}{P(B)}$
where the left hand term denotes the “probability of A given B” (and P denotes “probability of”). The top term on the fraction is “A intersection B”, which simply means that both A and B occur.

I will denote by X the “event” that Mr Smith said ‘yes’ to the at least one boy born on Tuesday question (the inverted commas are there because “event” is actually a technical term in probability). The probability we want to calculate is

$P(BB | X) = P(BB) \frac{P(X | BB)}{P(X)}$

Starting with the conditional probability on the top of this fraction, if we have a two-boy family, the answer to “do you have a boy” will certainly be “yes”, so we need to know the probability of at least one of the boys having a Tuesday birth date. There are 7 possible birthdates for each child, giving 49 possibilities. Of these, 7 have a Tuesday for the elder child and 7 for the younger, but this double-counts the case where both were born on a Tuesday, so we have:

$P(X|BB) = \frac{13}{49}.$

One way to calculate the probability of X itself is to break it down into different possible gender combinations:

$P(X) = P(X \cap BB)+P(X\cap GG)+P(X\cap BG)+P(X\cap GB).$

Here I am using the fact that probabilities of “disjoint” (non-overlapping) events add up, i.e. $P(A \cup B) = P(A) + P(B)$ if A and B are disjoint. Using the conditional probability formula, this gives:

$P(X) = P(X | BB) P(BB)+P(X | GG) P(GG) +P(X | BG) P(BG)+P(X | GB) P(GB).$

Now the probability of the boy in a mixed gender family being born on a Tuesday is 1/7 and the probability of having boy-girl or girl-boy are both genders is 1/4. Combining this with the fact that the probability of a boy born on Tuesday is zero in a two-girl family and what we already know about a boy-boy family know gives us

$P(X) = \frac{13}{49}\times \frac{1}{4} + 0 + \frac{1}{7}\times\frac{1}{4} + \frac{1}{7}\times\frac{1}{4}= \frac{13 + 2\times 7}{49}\times\frac{1}{4} = \frac{27}{49}\times\frac{1}{4}$

Putting this all together, we have

$P(BB | X) = \frac{1}{4}\times\frac{13}{49}/(\frac{27}{49}\times\frac{1}{4}) = \frac{13}{27}.$

While you are waiting for the next post with an alternative interpretation, you might want to think about Gardner’s two boy problem a bit more. In order to get to the classical conclusion that there is a 1/3 chance Mr Smith has two boys then you effectively have to assume that if he had one boy and one girl, he would definitely say ‘I have two children and at least one of them is a boy’ and not ‘I have two children and at least one of them is a girl’. Does that really make sense?

UPDATE: Here is a spreadsheet which simulates the classic version of the Gardner problem (i.e. assuming that you ask Mr Smith the question), and here is an alternative analysis.

# More on “Five Down”

Yesterday’s puzzle “Five Down” stimulated a fair amount of discussion both in the post’s comments section and via email. I also exchanged emails on the topic with the author of Futility Closet (which is where I came across the puzzle) and he told me that the puzzle generated a lot of correspondence for him too.

All the commenters on the blog came up with the correct solution, but there are quite a few different ways of looking at the problem, all of which help provide insight into the nature of money. Since that is a common topic for this blog, I will consider some of these perspectives here.

First, the solution itself. The question asked was “What was lost in the whole transaction, and by whom?”. Taking the “whole transaction” to include the banker finding the counterfeit note in the first place, the answer is that no-one lost anything, subject to a couple of assumptions. These assumptions are that the banker actually owns the bank and so the bank’s gains or losses are his gains or losses (otherwise we would have to conclude that the banker was up $5 and the bank was down$5), and that the banker and his wife pool their finances (so we treat her debt with the butcher as his debt).

The first way to think of the problem is a variation of the comment from James. Imagine that the $5 was not counterfeit at all and all the same transactions took place with a genuine note. But then imagine that when the banker closed the bank at the end of the day, taking notes and coins back to his safe, the$5 slips from his hands and is blown into the fireplace. There it is quickly consumed by the fire. Earlier in the day, the banker had a windfall of $5, but then he lost the same amount to the fire. He gained in the morning, lost in the evening and, although perhaps disappointed to have lost the$5 again, he was even on the whole transaction. No-one else involved lost either as they had simply performed legitimate transactions, clearing various debts, using a valid $5 note. The question now is, how is anyone any better or worse off in this scenario than if the note had been counterfeit all along? The answer is, they are not. Now that approach gives the right overall answer, but it may be unsatisfying to some as it doesn’t take account of the fact that a whole series of “invalid” transactions took place with the counterfeit note. This too can be clarified. If the note had been real, then the banker made a gain when he found the note, but finding a counterfeit note involves no gain, because it is worthless. In that case, the gain for the banker comes when he is able to discharge his debt with the butcher using a worthless note. So, he is still ahead early in the day, but the timing is slightly different. With a real note, the gain is in the finding and the transaction with the butcher is a neutral fair trade (legitimate$5 in exchange for a discharged debt). With a counterfeit note, the gain is delayed to the next step in the sequence. Of course, in receiving the counterfeit note, the butcher makes a loss. But then the butcher makes a gain when he in turn is able to discharge his debt to the farmer with a worthless note. And so on. Each person in the chain loses when they receive the $5 but has an offsetting gain when they use it to settle a debt, leaving them whole on the transaction. The chain continues all the way back to the bank, which loses$5 when the laundry woman settles her debt with the dodgy note. Assuming, as we are, that the bank’s loss is the banker’s loss, this simply offsets the gain the banker had when first paying the butcher. Again, everyone comes out even. Of course, if someone other than the banker had been left with the note, they would have been down $5 and the banker up$5. Having the transactions complete a full circle is a key part of the puzzle.

The final perspective is a more technical one. At the heart of money is the notion of a debt. Money is essentially a more convenient way of managing debts. If I buy a cow from a farmer and sell a meat pie to a patron at my restaurant, we could simply agree to record various debts: I owe the farmer one cow, the diner owes me one cow. Of course, this is inconvenient (not to mention risky) as we all have to maintain records denominated in a whole range of different commodities and I don’t really want to discharge my debt to the farmer by giving him a cow back. He has plenty already. Nevertheless, this points to the origins of money. In the excellent (if lengthy) treatise “What is Money” is it observed that “for many centuries, how many we do not know, the principal instrument of commerce was neither the coin nor the private token, but the tally”. Indeed in the Five Down puzzle, there are a whole string of tallies. Each of the players in the story has kept track of a debt due to them and one they owe to another. If the merchant did not owe the laundry woman but instead owed $5 to the farmer, the merchant and the farmer could simply agree to cancel their debts to one another. It is not so easy when the debts extend in a longer chain. Nevertheless, if one were to assemble all the parties in a single room and ask them all to consider their respective debts discharged, they should all readily agree. After all, they all owe$5 and all are owed $5 and it is much easier for everyone if that effective net zero position could really be zero without the fuss of worrying about chasing debts. It would be different if someone was owed more (or less) than they owed. We might call this simultaneous discharging of all the debts “multi-lateral debt netting”. In theory it is very attractive, but in practice we cannot get everyone in the same room to get it done. Effectively, the counterfeit note serves the purpose of facilitating multi-lateral debt netting. Because everything nets out evenly in the story, the counterfeit note can achieve the netting just as effectively as real money. The extra feature real money offers is that if the netting does not quite even out, those owed more than they owe can hang on to the money and use it for netting again in the future. Not so with the counterfeit money: once it is discovered, it loses its power to work. The solution to the puzzle lies in the fact that no debts were left over. I will end this discussion by reprinting a very similar story that one of my email correspondents sent to me (as I understand it, it is not new but has been updated to fit the times). It’s a slow day in a dusty little Australian town. The sun is beating down and the streets are deserted. Times are tough, everybody is in debt, and everybody lives on credit. On this particular day, a rich tourist from down south is driving through town , stops at the local motel and lays a$100 bill on the desk saying he wants to inspect the rooms upstairs in order to pick one to spend the night in.

He gives him keys to a few rooms and as soon as the man walks upstairs, the owner grabs the $100 bill and runs next door to pay his debt to the butcher. The butcher takes the$100 and runs down the street to repay his debt to the pig farmer.

The pig farmer takes the $100 and heads off to pay his bill at the supplier of feed and fuel. The guy at the Farmer’s Co-op takes the$100 and runs to pay his drinks bill at the local pub.

The publican slips the money along to the local prostitute drinking at the bar , who has also been facing hard times and has had to offer him “services” on credit.

The hooker rushes to the motel and pays off her room bill to the motel owner with the $100. The motel proprietor then places the$100 back on the counter so the rich traveller will not suspect anything.

At that moment the traveller comes down the stairs, picks up the \$100 bill, states that the rooms are not satisfactory, pockets the money, and leaves town.

No one produced anything. No one earned anything.

However, the whole town is now out of debt and looking to the future  with a lot more optimism.

And that, ladies and gentlemen, is how the Australian Government’s stimulus package works!!!

# Five Down

One of my favourite blogs is Futility Closet, which is sadly appropriate given its tagline “An idler’s miscellany”. This week it featured a puzzle called Five Down devised by the English mathematician Henry Dudeney. Since the subject of the puzzle is money, it seems like an appropriate one to share here on the Mule.

A banker in a country town was walking down the street when he saw a five-dollar bill on the curb. He picked it up, noted the number, and went to his home for luncheon. His wife said that the butcher had sent in his bill for five dollars, and, as the only money he had was the bill he had found, he gave it to her, and she paid the butcher. The butcher paid it to a farmer in buying a calf, the farmer paid it to a merchant who in turn paid it to a laundry woman, and she, remembering that she owed the bank five dollars, went there and paid the debt.

The banker recognized the bill as the one he had found, and by that time it had paid twenty-five dollars worth of debts. On careful examination he discovered that the bill was counterfeit. What was lost in the whole transaction, and by whom?

I will not reveal the solutiuon here to give you a chance to think about the puzzle. What I will reveal is that the “solution”, originally published in The Strand in 1917, was re-published on the blog yesterday but it is in fact incorrect! Understanding what is wrong with the original solution (and the blog’s author was quick to provide an update following feedback from his readers) gives some insight into two of the roles money plays: a medium of exchange and a store of value.