n! mod 15 = 0 for
Therefore, you just need to look at 1!+2!+3!+4! = 1+2+6+24 = 33
33 mod 15 =
Each term k!, where k >= 5, is divisible by 15 without a remainder, since it contains factors 3 and 5. THEREFORE, the sum 1! + 2! + 3! + . . . + 95! gives the same remainder when is divided by 15, as 1! + 2! + 3! + 4!, which is equal to 1 + 2 + 6 + 24 = 33. The remainder of 33 when divided by 15 is 3. THEREFORE, the ANSWER to the problem's question is 3.