What does this question test?
It asks for the number of zeros 100! ends with. If a number ends with zero it has to be a multiple of 10. So essentially we have to find out how many times in 100! does multiplication by 10 happen. But multiplication by 10 also happens when multiplication by both 5 and 2 happen. So it is better to see the factors of the number and then find the number of times multiplication by all those factors happen.
Consider the simpler example. 4*5*6. How many zeros does it end with?
We see 2 occurs twice in 4 which is 2*2, and once in 6 which is 2*3 and 5 occurs only once. So 5 is a limiting factor. Since 5 occurs only once, the number of times multiplication by both 5 and 2 happen or in other words the number of times multiplication by 10 happens or the number of zeros the number ends with is only 1.
In the case of 100!, 5 occurs in 5, 10, 15, 20 and so on up to 100 i.e, 20 times. But remember 5 occurs twice in 25 which is 5*5 , twice in 50 which is 5*5*2 and similarly twice each in 75 and 100. So it actually occurs 24 times.
2 occurs a lot more times and so 5 is the limiting factor.
Applying the same logic as in the simpler example, the number of zeros 100! ends with is 24.
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