question from Gmatprep test1

banona · Posted: Sat Mar 03, 2007 12:22 pm Post subject: question from Gmatprep test1

Can somebody tell me what the most efficient way to solve this kind of DS GMAT questions ;

the question says , if the remainder of the division of (n-1)*(n+1) by 24 is r, what's the value of r ?
1- n is not divisible by 2
2- n is not divisible by3

I appologize if the question rose somewhere else here in this forum while,butI didn't see it.

Thanks

jayhawk2001 · Posted: Sun Mar 04, 2007 7:36 pm Post subject:

Not sure if this is the most efficient way but here goes...

1 - insufficient. Take n = 1 and n = 3 for example.
They yield remainders of 0 and 8 resp. No unique value. So rule out 1.

2 - insufficient. Take n = 1 and n = 2.
They yield remainders of 0 and 3 resp. No unique value. So, rule out 2

A number that is not divisible by 2 AND not divisible by 3 can be
written as n = 6x + 1 or n = 6x - 1

For n = 6x+1
(n-1)*(n+1) / 24
= 6x * (6x+2) / 24
= 12x * (x+1) / 24

We know that x*(x+1) will be even or 0, so the above equation will
always yield a remainder of 0.

Similarly for n=6x-1
(n-1)*(n+1) / 24
= (6x - 2)*6x
= 12x*(x-1) / 24

Again, x*x(-1) will be even or 0 and hence remainder r will be 0.

So, C looks like the correct answer.

On the actual GMAT, in the interest of time, it might be worthwhile
trying n = 1, 3, 5, 7, 9 etc. We can see that the remainder is 8 when n
is not divisible by 3 and remainder is 0 when n is divisible by 3 (for
all values n > 0). Using this pattern, I guess we can settle on C.

banona · Posted: Mon Mar 05, 2007 6:50 am Post subject:

thank you,
I would go for picking numbers on the real GMAT too.

rajesh_ctm · Posted: Tue Apr 03, 2007 7:44 pm Post subject:

jayhawk2001, that is a wonderful explanation! Thank you. Gives us a valuable technique, we can use for similar questions.