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Question

What is the least value of "n" if 1*2*3...*n ends with 6 zeroes?
1) 30
2) 25
3) 5^6
4) 36
5) 24

Can someone explain how to solve a question of this kind.

The number of zeroes with which a number ends is a function of the number of '5's and the number of '2's.

As there will be lesser number of 5s than the number of 2s, it will suffice if one looks at the number of 5s.

If you need 6 zeroes, you need to go up to such a number when six 5s can be counted.

The first 5 appears at 5, the second at 10, the third at 15, the fourth at 20, the fifth and sixth at 25. 25 is 5^2 and hence has two 5s.

Hence, the value of 'n' is 25. Choice B

Thanks ksb.

Is there an alternative to actually listing it this way?

There is no other alternative explanation for this.