Problem Solving

If n is an integer greater than 6, which of the following must be divisible by 3

a,n(n+1)(n-4)
b,n(n+2)(n-1)
c,n(n+3)(n-5)
d,n(n+4)(n-2)
e,n(n+5)(n-6)

I see both A and E are correct by plugging number 7 and 9. is this question wrong, which is from 15 test set.

duongthang wrote:If n is an integer greater than 6, which of the following must be divisible by 3

a,n(n+1)(n-4)
b,n(n+2)(n-1)
c,n(n+3)(n-5)
d,n(n+4)(n-2)
e,n(n+5)(n-6)

I see both A and E are correct by plugging number 7 and 9. is this question wrong, which is from 15 test set.

Hi,

Once you have narrowed down to options A and E, you can try plugging in more numbers.
For 11, the option E is not divisible by 3, but A is, thus A is the answer.

The question basically needs us to find an answer in which the expression is divisible for any n>6, and that holds true only for A.

HTH's,
Vivek

duongthang wrote:If n is an integer greater than 6, which of the following must be divisible by 3

a,n(n+1)(n-4)
b,n(n+2)(n-1)
c,n(n+3)(n-5)
d,n(n+4)(n-2)
e,n(n+5)(n-6)

I see both A and E are correct by plugging number 7 and 9. is this question wrong, which is from 15 test set.

q is correct try with 11 E is not satisfying

Subsitution can take time here. What is the correct/ logical way of doing the question.

Thanks

Hi crackgmac,

I would actually go with plugging in numbers here.

But the question is asking for something that MUST be divisible by 3. If we plug in numbers into an answer choice and it works, we have only proven that the expression in the answer choice COULD be divisible by 3 (not that it MUST be).

But if we plug in numbers and it doesn't work, we have proven that it does NOT HAVE TO BE divisible by 3, and so can eliminate.

In other words, here, you can plug in numbers to eliminate choices (you can prove that they could be false--that they don't HAVE to be divisble by 3). But if you plug in a number and it works, you have only proven that it could be true--that it could divide 3, not that it must.

So you simply plug in numbers until you are able to elminate all the answer choices. Again, here, you can't plug in numbers to select an answer choice (if it works, you keep it in the running, and if it doesn't you eliminate).

Remember that the best approach on the GMAT is not necessariy the same as some "correct" approach you would have to us in a show-your-work exam. The testmaker wants to reward testtakers who use alternative and more efficient reasoning techniques.

duong--b/c this is a released GMAT question, it cannot be "wrong".

If the question is a released GMAT question or from a major test prep company, you should always assume the question is "right".

duongthang wrote: a,n(n+1)(n-4)
b,n(n+2)(n-1)
c,n(n+3)(n-5)
d,n(n+4)(n-2)
e,n(n+5)(n-6)
which is from 15 test set.

Consider n=2m, i.e. an even no:
so I can say : n=3m+1, or n=3m-1
a: n = 3m+1 (3m+1)(3m+2)3(m-1): multiple of 3
or n = 3m-1 (3m-1)(3m)(3m-4) : Multiple of 3
e: n = 3m+1 (3m+1)3(m+2)(3m-5): Multiple of 3
n = 3m-1 (3m-1)(3m+4)(3m-7) : Not guaranteed

SO A: