I posted a thread about modular math but this has a different application.
Suppose that a*b = 0 (mod n), and that a and b are positive integers both less than n.
Does it follow that either a|n or b|n? If so, why? If not, is there an example providing the contrary?
The notation means that ab is divisible by n, or n | ab, which means there is an integer m such that ab=mn. But you can't conclude that a | n or b | n, since some of the prime factors of a and b might be contained in m.
As Bruno J. said, it's easy to find counter examples. Here's one:
a=6, b=5, m=10, n=3: ab=30, so and neither 6 nor 5 divides 3.