Puzzles, Maths and Algorithms: Finding First Non-Zero Digit of N Factorial

Puzzles, Maths and Algorithms: Finding First Non-Zero Digit of N Factorial

ind the first non-zero digit of N!. Assume N! to be extremely large.

Solution:
If we assume that the product doesnt contain either of 2's and 5's, then finding last digit is trivial. Simple multiply the result with next number and take its mod 10. The problem is complicated as 2's and 5's lead to trailing 0's. So the first step is to remove 2's and 5's. Then solve the problem. The cancel 5's with 2's and multiply back the left over 2's.

For Ex. 7! = 1 * 2 * 3 * 4 * 5 * 6 * 7. We extract 2's and 5's so the product is 1 * 3 * 3 * 7. The trailing digit is 3. We have extracted four 2's and one 5. So we get 3 extra 2's. So multiply to 3 we get 4 as the answer. To check 7! = 5040

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