Annoying Math Riddle

This riddle is really bugging me:

“Yes, but if we assume that there is a 10 at the top and 5 and 4 before 3, then the result is different.”

Why do you have to be assuming anything at all? That’s why I always sucked at math. I don’t have the imagination for it. In literary criticism we work with what there is. Nobody says, “but assume Jane Eyre eventually dumped Mr Rochester, built a spaceship, and traveled to another galaxy. Then it would be a completely different genre!”

26 thoughts on “Annoying Math Riddle

    1. xykademiqz’ answer is probably what was intended. But you get 18 if you solve it using the geometric arrangement of the numbers on the page– i.e. each answer is the product of the number before the =, and the number directly above it. Since we are given no rules or instructions, I hold both answers are valid 😉

      Liked by 2 people

          1. “But then where does the 90 come from in that explanation?”

            Either way (including “4” and “3” in the list or not) doesn’t the “90” come from an assumed “10” above the “9”?


            1. Well, yes. But since the set comes with no instructions, the writer uses the “equals” sign improperly (but still labels it a MATH test when it is no such thing), and we are given no rules or parameters… I feel perfectly fine assuming a ten there, because of the 90 on the first line. What if they simply neglected an ellipsis between the “6” line and the “3” line? It would fit with the overall carelessness of the thing, as it is set up.

              Liked by 1 person

      1. One would hope that even in something like a riddle or a logic problem, the equal sign would be used correctly. What’s next? People claiming 2+2=5? Oh wait a minute, that already happened….


    1. It helps to think of it as equivalence rather than arithmetic equality. A map between two sets that connects the elements of the two sets 1-to-1 uniquely both ways (in math it’s called a bijection). So for the connection between 7 and 56 you can write as “=” because 7–>56 and 56–>7 and neither will correspond to any other natural number.


    2. You’re right—should be arrows instead of equal signs. My math proofs professor would probably throw a pencil at the person who wrote it that way. :p

      Liked by 1 person

    1. “Jane Eyre DIDN’T build a spaceship?”

      Don’t beat yourself up, it’s only in the super unabridged version written after Charlotte had some bad fish…. they end up opening a drag show on the moon as well and then set the universe on fire while fighting the asteroid monsters. An increasingly large body of scholars are arguing for it to be included in the definitive version.


  1. I got 18 on my own, but I immediately went to team 12 as soon as I saw the option.

    I’m not sure mathematicians use the term, or at least that they use it this way, but what makes math fun is its universality – the rules operate independently of their content. If you want to calculate the area of a 5 by 5 square like a sane person, you can. If you want to do a sane variation of the same, like 4 by 6 or 3 by 7, you can do so as well using the same rules. And if you want to do the same for something stupid, like finding out the area of half a cat’s whisker stretched from horizon to horizon, you can do that too, using the same rules.

    While a number of patterns can reasonably be matched to the couplings we see here, the n(n+1) is the most in tune with the spirit of math since it is the most universal – you could use it to produce pattern-matching couplings from fractions, or imaginary numbers or whatever else you want. Answering 12 gives yo a tool that makes infinity your whipping boy, answering 18 just gives you an answer to the question as posed.

    Which of course means that answering 18 is much kinder to infinity.


    1. Traitor.

      🙂 🙂

      You capture the conflict perfectly. I’m very literal, so I answer the question exactly as asked. This desire to extrapolate into the infinity scares me.

      I can’t wait to try this on my mathematician husband. What if he says 12? What does it mean for our relationship??


      1. I’m sure he will say 12. But since this is not a well defined math problem, I can’t say that 18 or answer you could make a reasonable argument for is wrong strictly speaking.


  2. Are you familiar with Jasper Fforde’s Eyre Affair? In addition to Jane Eyre going to India, the book also involves time travel and the Crimean War still being fought in the 1980s. :p


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