Gmatbarcelona CAt - Integers

molt_llest · Posted: Sun May 11, 2008 12:45 am Post subject: Gmatbarcelona CAt - Integers

Thank you for your help.

If the number X is equal to the product of the first n positive integers {1,2,3...n}, what is the minimum value of n such that the last six digits of x are equal to 0?

a.- 20
b.- 25
c.- 30
d.- 35
e.- 40

Correct answer: b (answer in white color, drag over to see the answer)

molt_llest · Posted: Mon May 12, 2008 5:50 am Post subject:

Anyone knows how to do it?

netigen · Posted: Mon May 12, 2008 9:46 am Post subject:

To get the 0s in the last digit you need to multiply a number by 10

10 has two factors 2 x 5

which means you need to figure how many 5s and 2s will make up for 6 trailing zeros

Note the factor 2 can come from any even but 5 can only come from a number ending in 5 or a number ending in 0

Akshat's asnwer below is correct.

akshatsingh · Posted: Mon May 12, 2008 9:30 pm Post subject:

2 x 5 x 10 = 2 zeros
2 x 15 x 20 = 2 zeros
25 x 4 = 100 = 2 more zeroes

The answer should be 25. B

molt_llest · Posted: Tue May 13, 2008 6:48 am Post subject:

Thank you for your answer, I realize now the way to get it. Thank you!