100 factorial solution I think the proposed solution ( http://www.techinterview.org/Solutions/fog0000000150.html )  is wrong. Both factors of 10 and factors of 5 squared are also factors of 5. So the only number that really matters is the number of factors of 5. Which are 5,10,15,20,25,30,35,40,45,50,55,60,65,70,75,80,85,90,95,100. For a total of 20 factors of 5, giving 20 trailing zeros. Jonno Sunday, March 14, 2004 whoops. when I said 'factor of 5' above, i meant 'multiple of 5' Jonno Sunday, March 14, 2004 I believe the numbers listed, namely these numbers, 5,10,15,20,25,30,35,40,45,50,55,60,65,70,75,80,85,90,95,100. are indeed the numbers that "matters." And indeed, we want to count how many factors of 5 we have in those numbers, but the total is not 20, the total is 24 as given in the "proposed solution." Here is why The following numbers have two factors of 5 25 = 5 * 5 50 = 2 * 5 * 5 75 = 3 * 5 * 5 100 = 4 * 5 * 5 so total = 20 + 4 = 24 factors of 5 JHY Monday, March 15, 2004 24 is correct. To convince yourself, all you need to do is calculate 100! and count the zeros! 100! is 93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000 :-) Paul Viney Tuesday, March 16, 2004 or, using line breaks ... 93326215443944152681699238856266700 49071596826438162146859296389521759 99932299156089414639761565182862536 97920827223758251185210916864 000000000000000000000000 Paul Viney Tuesday, March 16, 2004 Yep. I am very wrong. How embarrassment. Jonno Monday, March 22, 2004   Fog Creek Home