If n = 2Ã—3Ã—5Ã—7Ã—11Ã—13Ã—17, then which of the following..

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swerve: Legendary Member; Posts: 2535; Joined: Sun Oct 29, 2017 2:04 pmFollowed by: 6 members

by swerve » Mon Dec 04, 2017 3:56 pm

If n = 2Ã—3Ã—5Ã—7Ã—11Ã—13Ã—17, then which of the following statements must be true?

I. n^2 is divisible by 600

II. n + 19 is divisible by 19

III. Is even (n + 4)/2 = even

A. I only
B. II only
C. III only
D. I and III only
E. None

The OA is E.

Please, can any expert explain this PS question for me? I have many difficulties to understand why that is the correct answer. Thanks.

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Source: Beat The GMAT — Problem Solving | Mon Dec 04, 2017 3:56 pm

Matt@VeritasPrep: GMAT Instructor; Posts: 2630; Joined: Wed Sep 12, 2012 3:32 pmFollowed by: 119 members; Location: East Bay all the way; Thanked: 625 times; GMAT Score:780

by Matt@VeritasPrep » Mon Dec 04, 2017 6:11 pm

We know that n = 2 * (some huge odd number). From there:

(i) For n to divide by 600, it must divide by 6 * 100, or 6 * 10 * 10, or 6 * 2 * 5 * 2 * 5. But we saw that n only has ONE factor of 2, and to divide by 600 it'd need THREE factors of 2, so (i) is NOT true.

(ii) if n + 19 = 19 * something, then we can say

n + 19 = 19 * something

n = 19 * something - 19

n = 19 * (something - 1)

So would be a multiple of 19, because it equals 19 times some number. But n ISN'T a multiple of 19: its odd factors only go up to 17, and 19 is prime. (ii) is NOT true.

(iii) If (n + 4)/2 = even, then

(n + 4) = 2 * even

n = 2 * even - 4

Now any even number is a multiple of 2, so we can rewrite "even" as "2 * something":

n = 2 * (2 * something) - 4

n = 4 * something - 4

n = 4 * (something - 1)

and this is false for the same reason that (ii) is false, n itself is NOT a multiple of 4.

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