Advanced GMAT

Expert replies

navdeepbajwa: Senior | Next Rank: 100 Posts; Posts: 74; Joined: Sun Aug 02, 2009 9:10 pmFollowed by: 1 member; Thanked: 1 times

by navdeepbajwa » Tue Oct 27, 2009 10:00 pm

If k is an integer, and 35^2-1/k is an integer, then k could be each of the following, EXCEPT

(A) 8(B) 9(C) 12(D) 16(E) 17

Like this post

Source: Beat The GMAT — Problem Solving | Tue Oct 27, 2009 10:00 pm

gopalkrishna.r: Newbie | Next Rank: 10 Posts; Posts: 1; Joined: Tue Oct 27, 2009 10:54 pm

by gopalkrishna.r » Tue Oct 27, 2009 10:59 pm

(D) 16 is the answer

How did I get this? 35^2 = 1225 (easy way is using vedic maths)..any number X5^2 would be X(X+1)25... so in this case, 3*4 25 which is 1225.

1225-1 = 1224

Now, actual problem is to check which of the given numbers is exactly divisible by 1224. From divisibility tests, 16 does not pass. 1224/16 = 72.5 and so it is the answer.

Like this post

GMAT/MBA Expert

Ian Stewart: GMAT Instructor; Posts: 2623; Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members; Location: Montreal; Thanked: 1090 times; GMAT Score:780

by Ian Stewart » Wed Oct 28, 2009 5:19 am

Or you can factorize 35^2 - 1; it's a difference of squares:

35^2 - 1 = (35 + 1)(35 - 1)
= 36*34
= 2^2 * 3^2 * 2 * 17
= (2^3)(3^2)(17)

from which we can see that 2^3 = 8, 3^2 = 9, (2^2)(3) = 12 and 17 are all divisors of 35^2 - 1, whereas 2^4 = 16 is not.

For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com

Like this post

mp2437: Master | Next Rank: 500 Posts; Posts: 177; Joined: Thu Aug 14, 2008 11:59 am; Thanked: 25 times

by mp2437 » Wed Oct 28, 2009 5:52 am

gopal, thanks for the little trick, never knew about that!

Nonetheless, still requires much work. I like Ian's style.

Like this post

navdeepbajwa: Senior | Next Rank: 100 Posts; Posts: 74; Joined: Sun Aug 02, 2009 9:10 pmFollowed by: 1 member; Thanked: 1 times

Re

by navdeepbajwa » Wed Oct 28, 2009 9:57 am

Thanks Ian

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review