GCD and LCM of 2 Numbers in C

Is Prime?

Generating Primes using Sieve of Eratosthenes

Prime Factorization

Ref: Programming Challenges

10070 - Leap Year or Not Leap Year and ...

Used divisibility rules instead of looping with taking the mod. 

For 400, test for divisibility by 100 and then test for divisibility by 4 (summing digits 3 & 4*(10))

Check for test cases on the web.