First, as I mentioned yesterday, we have just released the Night School Sleep music on iTunes.  This 30 minute piece is scientifically designed to help you fall asleep and can be downloaded here.

Second, today I am off to New York City to see the premiere of a new documentary about James Randi.  I have seen a rough cut and it looks jolly good.  You can see the trailer here. If you get a chance to see it, go watch.

Finally, here’s the puzzle.  Please do NOT post your answer, but do say if you think you have solved the puzzle and how long it took. Solution on Monday.

Imagine a mythical family in which each daughter has the same number of brothers as she has sisters. In addition, each son has twice as many sisters as he has brothers. How many sons and daughters are there in this mythical family..?

I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.


  1. A minute or so. I just brute-forced it, and I’m sure there’s an elegant way of doing it, but it’s a holiday so my brain is taking the day off

  2. Already it looks like 3/4 of the people answering this have missed the important stipulation in the question.

    It is a MYTHICAL family.

    There may be many more types of gender in a mythical family than in a real one. Chimeras, hemaphrodites, werepeople, centaurs, conjoined twins of alternating genders, people possessed by others of a different gender, conjoined centaurs, cis- and transgendered peoples and so on. Plus people who identify as one gender but are legally or socially assigned to another.

    A proper answer must not write off all these varieties of humanity. We no longer live in the dark ages were we can burn or purge or banish the so called non-neurotypical genitalia.

    Thankfully too we no longer live in a world where turning a cat upsidedown is acceptable entertainment.

    If you have answered already please reflect on our larger inclusive world and then reanswer again.

  3. I actually found two solutions: the one they’re expecting, and a rather trivial one that doens’t disagree with the wording of the puzzle.

  4. I do so miss the scratch & trash of multi-choice. And then those sixth grade years pay off again when I recall the dimly lit marbles of Ratio. I will not trifle w/ Algebra – you cannot make me, I am an Adult! 7 gut-wrenching minutes.

  5. I agree with Barry Goddard. A creative thinker will always see past the limitations of the question. When you solve a puzzle in a puzzle book or on a website like this, you’re supposed to solve the puzzle “within the problem,” that is, within a set of assumptions that aren’t actually put forth in the question. If you look past these assumptions, you find a whole new set of answers available to you. This might seem silly if you’re solving a problem in a puzzle book, since it’s not a real world problem to begin with, but it’s actually an important point, because when you attempt to solve real world problems, it’s important that you DON’T get entrenched in assumptions. Finding alternate solutions to world problems is actually a good exercise in looking past the cultural assumptions that real world problems fall into.

    That being said, there is an obvious “within the problem” answer to this question, which this time took me about ten seconds to find. Some of the Friday puzzles have been difficult for me, but I found this one incredibly easy and the answer presented itself to me without any effort.

  6. I think the wording below would have made the puzzle more interesting/fun:

    Three children are chatting. One says “I have the same number of brothers and sisters.” The second child says, “Not me. I have twice as many sisters as brothers.” The third child says, “How can that be? You’re both in the same family–you have the same parents!”. Assuming everyone is telling the truth, how can this be?

  7. Why does it have to be a mythical family? The answer is the exact same number of boys and girls as the invisible family that lives in my head.

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