This thread has 2 parts.
The first part consists of a problem I'm going to ask brains more intelligent than mine to help me solve.
The second part consists of my wanting to know whether a person who has a BA in math who is this helpless when it comes to proofs should bother with grad school.
The first part.
Prove that if
(x1 + x2 + ... + xn) % 3 = 0
where xi is an integer such that 0 <= xi <= 9
then the number whose digits equal those xi's, in that particular order, is also divisible by 3.
This relates to that shorthand way most of us learned in school to tell if a number is divisible by 3. For example, how do we tell if the number 23520 is divisible by 3? We add up its digits and if the sum is divisible by 3, then so is the number. In this case 2 + 3 + 5 + 2 + 0 = 12, which means that 23520 is also divisible by 3.
And now back to the second part of this post: given that I have a BA in math, and given that I spent almost 2 hours while I was driving on the highway trying to mentally solve this problem and couldn't do it, does it mean that I am not grad school material? Or am I trying to solve a problem that the average BA in math probably wouldn't be able to solve?