# A few questions

1. Find the number of pairs (m, n) such that 2m - 2n = 63 in which m and n are nonnegative integers.

2. What is the units digit of 625 - 324 ?

3. How many perfect squares divide the number 4!*5!*6! ?

4. What is the last digit of the sum 1! + 2! + 3! + ... + 2010! + 2011! ?

Thanks!