Chinese Remainder Theorem
The Chinese Remainder Theorem is a number theoretic result. It is one of the only theorems named for an oriental person or place, due to the closed development of mathematics in the western world.
Applicability
Much like the Fundamental Theorem of Arithmetic, many people seem to take this theorem for granted before they consciously turn their attention to it. It ubiquity derives from the fact that many results can be easily proven mod (a power of a prime), and can then be generalized to mod 
using the Chinese Remainder Theorem. For instance, Fermat's Little Theorem may be generalized to the Fermat-Euler Theorem in this manner.
General Case: the proof of the general case follows by induction to the above result (k-1) times.
Extended version of the theorem
Suppose one tried to divide a group of fish into 
, 
and 
parts instead and found 
, and fish left over, respectively. Any number with remainder mod must be odd and any number with remainder mod must be even. Thus, the number of objects must be odd and even simultaneously, which is a contradiction. Thus, there must be restrictions on the numbers 
to ensure that at least one solution exists. It follows that:
The solution exists if and only if 
for all 
where 
stands for the greatest common divisor. Moreover, in the case when the problem is solvable, any two solutions differ by some common multiple of 
. (the extended version).
