The one thing I hated about maths in school and university was the fact that I had to show my working. Of course I knew that it helped the marker see that you understood the problem, but I just found it incredibly tedious. Particularly when I knew the answer right after reading the question.
I recall back in university I was asked many times to find the greatest common divisor (GCD) of two integers using Euclid's algorithm. This is basically the exact situation I described above, you can work out the greatest common divisor in your head fairly easily with a smallish number but to actually show your working can take a quite a bit of writing.
Find the greatest common divisor of 108 and 30
108 = 30 x 3 + 18
30 = 18 x 1 + 12
18 = 12 x 1 + 6
12 = 6 x 2 + 0
^
gcd(108, 30) = 6
So after doing it a couple of times I went ahead and spent a few minutes writing a little program that solved the problem with working shown. I lost the original source but have reproduced it for a little fun. :)