Can someone help with this question from the free software?

Expert replies

simunda: Newbie | Next Rank: 10 Posts; Posts: 5; Joined: Sun May 02, 2010 12:39 pm

by simunda » Sat Nov 06, 2010 9:34 am

Hope I'm not breaking any rules here but I have come across this question a couple of times on the free official software CATs and just cannot work out how the answer is reached. Can anyone help?

What is the greatest prime factor of 4 to power 17 minus two to power 28?

Thanks.....

Like this post

Source: Beat The GMAT — Problem Solving | Sat Nov 06, 2010 9:34 am

Bharat: Senior | Next Rank: 100 Posts; Posts: 70; Joined: Sun Jul 22, 2007 11:43 pm; Thanked: 8 times

by Bharat » Sat Nov 06, 2010 9:45 am

Answer: 7

4^17 - 2^28
= 2^34 - 2^28
= 2^28 ( 2^6 - 1)
= 2^28 (64-1)
= 2^28 (7*9)
= 2^28 * 3^2 * 7 --> 7 is the largest prime here.

Regards.

Like this post

simunda: Newbie | Next Rank: 10 Posts; Posts: 5; Joined: Sun May 02, 2010 12:39 pm

by simunda » Sat Nov 06, 2010 9:59 am

Thanks , that helps a lot. I kept getting the first few steps but got the difference to be 2^6.....could you explain why its 2^6-1 please?

Like this post

Bharat: Senior | Next Rank: 100 Posts; Posts: 70; Joined: Sun Jul 22, 2007 11:43 pm; Thanked: 8 times

by Bharat » Sat Nov 06, 2010 10:19 am

Bharat wrote:Answer: 7

4^17 - 2^28
= 2^34 - 2^28
= 2^28 ( 2^6 - 1)
= 2^28 (64-1)
= 2^28 (7*9)
= 2^28 * 3^2 * 7 --> 7 is the largest prime here.

Regards.

2^34 = 2^28 * 2^6 [exponents of the same base are added during multiplication]
revisit the 2nd line of the answer again:
2^34 - 2^28
= 2^28 * 2^6 - 2^28 * 1
=2^28 (2^6 - 1) [2^28 is common]
Let me know if this does not help.

Like this post

simunda: Newbie | Next Rank: 10 Posts; Posts: 5; Joined: Sun May 02, 2010 12:39 pm

by simunda » Sat Nov 06, 2010 12:00 pm

perfect, thank you so much, got it now!

Like this post

zachthegnome: Junior | Next Rank: 30 Posts; Posts: 10; Joined: Sun Nov 07, 2010 8:02 am; Thanked: 1 times

by zachthegnome » Mon Nov 08, 2010 9:11 pm

I am still missing something on the 2^6-1 step.

Please elaborate further. Thanks.

Like this post

Bharat: Senior | Next Rank: 100 Posts; Posts: 70; Joined: Sun Jul 22, 2007 11:43 pm; Thanked: 8 times

by Bharat » Mon Nov 08, 2010 9:41 pm

zachthegnome wrote:I am still missing something on the 2^6-1 step.

Please elaborate further. Thanks.

I have used the following two formulas:
1. a = a*1
2. a*b - a*1 = a (b - 1)

In these, replace "a" with "2^28" & "b" with "2^6"; & you will get the requisite step.

Let me know if there are any ambiguities. Thanks.

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

559 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