[sltkr]

> the statement "you can do it in your head" generally does not entail this much complexity

It's funny that you jump to accusing OP of falsely claiming you can do it in your head, without apparently considering the alternative: that the intended solution is a simpler one than you outlined.

Trust me, you can do this in your head if you know basic high school level math, and you don't need to solve quadratic equations or keep a ton of numbers in your head at the same time.

If I ask you if 123456789 is a prime number, do you complain that it's not fair to make you perform division on such a long number?

[fsckboy]

>Trust me, you can do this in your head if you know basic high school level math

yeah, i guess it was a mistake to graduate from MIT undergrad and grad school in quant fields, i should have just stuck with high school math

>If I ask you if 123456789 is a prime number, do you complain that it's not fair to make you perform division on such a long number?

you tell me, is 13717421 prime?

[dhosek]

The difference between the two is that it’s clear that 123456789 can’t be prime since the sum of the digits is a multiple of 3, which doesn’t even require finding the sum since we know 1+8, 2+7 up to 4+5 are multiples of 3. I can even tell you that 43717421 isn’t prime without having to do a divisibility test on it by looking at the digits, although it is a bit more tedious than the 123456789 field.

[fsckboy]

the difference between the two is that I removed the factors of 3 from 123456789 to get 13717421. so much for your secret knowledge of a hyperspecific case.

[sltkr]

You're still missing the point of these problems, which is to challenge you to come up with a clever proof rather than brute-force the solution.

dhosek understood the assignment by making an argument that 123456789 is composite without relying on explicit division of a 9-digit number, which most people would find rather difficult to do in their heads.

Similarly, the posted link is about tiling a mutilated chessboard with dominos. Tiling problems in general are NP-hard, so clearly this isn't something you can solve in your head _in general_, but the charm of that specific problem is that you _can_ solve it by making an insightful observation to avoid the brute force computations.

Similarly, for the puzzle you complained about: we are asked to find 1/a + 1/b where a × b = 37 and a + b = 18. The general solution is to solve a system of two linear equations which involves solving a quadratic equation, which is possible, but tedious and difficult to keep in your head, but the entire point of the question is that there is a better way to figure out the result.

[lupire]

That's only 9 digits. Determining if 12345678910 is primes would be outrageous. That's got more digits than I have fingers!