3. How many zeros at the end does the number 2022! have? (n! is a product of all positive integers from 1 to n. 2! = 1 x 2, 3! = 1 x 2 x 3, etc.) Can't use the calculator or any other electronic help.

4. Prove that the square of every prime number other than 2 and 3 is congruent to 1 mod 12.