Elementary number theory

• What Are the Last Two Nonzero Digits of 2016!?
  The last two nonzero digits of 2016! are 64. Strip its factors of 2 and 5, remove trailing-zero pairs, then reduce the result modulo 100.
• What Is the Greatest Common Divisor of Prime Numbers?
  The gcd of two distinct primes is 1; if they are the same prime, their gcd is that prime itself.