Solve for x The book say when . Why is this? And how am I supposed to use this to solve this problem?
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Originally Posted by dwsmith Solve for x The book say when . Why is this? And how am I supposed to use this to solve this problem? 5 = 5 * 4 * 3 * 2, you can see why it's divisible by 10 and 15? Also 6! = 6 * 5! so it's also divisible by 10 and 15, etc. But the mod 15 part really doesn't matter here. So 1! + 2! + ... + 1000! looks like 1 + 2 + 6 + 4 + 0 + 0 + 0 + ... + 0 (mod 10).