GMAT in 3 days! Experts please help explain answer

This topic has expert replies
Newbie | Next Rank: 10 Posts
Posts: 4
Joined: Fri Jan 04, 2013 11:55 pm
First let me say thank you all for helping me in the past 3 weeks! I spent each day on these forums reading every post and comment that was available, increasing my practice score from 660 to 740. I decided to make a username for these last few days to seek help on a few questions. I will greatly appreciate any advice on my posts, and I promise to always use spoilers!


Image
[/img]
Source: — Problem Solving |

Newbie | Next Rank: 10 Posts
Posts: 4
Joined: Fri Jan 04, 2013 11:55 pm

by sharpdresser » Sat Jan 05, 2013 12:18 am
OA D-7

Newbie | Next Rank: 10 Posts
Posts: 4
Joined: Fri Jan 04, 2013 11:55 pm

by sharpdresser » Sat Jan 05, 2013 1:48 am
OA D-7

Senior | Next Rank: 100 Posts
Posts: 65
Joined: Wed Mar 28, 2012 6:54 pm
Location: Canada
Thanked: 15 times
Followed by:1 members

by Param800 » Sat Jan 05, 2013 3:46 am
Hello There,

I did this question this way--

4^17-2^28 = 2^34-2^28

Now factor out 2^28 from the above expression...

2^28 ( 2^6 -1 ) = 2^28 * 63

63 can be written as 7*3*3

...So from above we know that the greatest prime factor is 7.

Hope this helps. And BEST OF LUCK for your Exam. :)

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 2095
Joined: Tue Dec 04, 2012 3:22 pm
Thanked: 1443 times
Followed by:247 members

by ceilidh.erickson » Sat Jan 05, 2013 7:47 am
Param800 wrote:Hello There,

I did this question this way--

4^17-2^28 = 2^34-2^28

Now factor out 2^28 from the above expression...

2^28 ( 2^6 -1 ) = 2^28 * 63

63 can be written as 7*3*3

...So from above we know that the greatest prime factor is 7.

Hope this helps. And BEST OF LUCK for your Exam. :)
This is absolutely correct. Just a general piece of advice with any exponent problems like this (particularly ones that mention "factors"): always start by getting down to prime factorization. Convert 4 to 2^2, etc. This will allow you to get equal bases, and then to simplify.

Good luck!
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education