The answer to life, the universe, and everything
This takes about 75s on a raspberry pi overclocked to 1GHz

#!/usr/bin/env python
from itertools import product, count

for n in count(1):
    i = 0
    for a, b, c, d in product(range(n), repeat=4):
        if a &gt; b &gt; c &gt; d &gt; 0 == (a*b-c*d)%n == (a*c-b*d)%n == (a*d-b*c)%n:
            i += 1
    if i == n:
        break
print i


It works because:


  42 is the only known value that is the number of sets of four
  distinct positive integers a,b,c,d, each less than the value itself,
  such that ab-cd, ac-bd, and ad-bc are each multiples of the value.
  Whether there are other values remains an open question


http://www.mathpages.com/home/kmath255.htm