Exponents

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heshamelaziry: Legendary Member; Posts: 869; Joined: Wed Aug 26, 2009 3:49 pmFollowed by: 3 members; Location: California; Thanked: 13 times

by heshamelaziry » Mon Sep 21, 2009 3:33 pm

The value of (2^-14 + 2^-15 + 2^-16 + 2^-17)/5 is how many times the value of 2^-17 ?

The answer seems to have a flaw in multiplying fractions !!!

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Source: Beat The GMAT — Problem Solving | Mon Sep 21, 2009 3:33 pm

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

Re: Exponents

by Brent@GMATPrepNow » Mon Sep 21, 2009 6:00 pm

heshamelaziry wrote:The value of (2^-14 + 2^-15 + 2^-16 + 2^-17)/5 is how many times the value of 2^-17 ?

The answer seems to have a flaw in multiplying fractions !!!

For some reason, I'm having a hard time adding an image to the post. So, I've attached the solution.

Cheers,
Brent

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Brent Hanneson - Creator of GMATPrepNow.com

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heshamelaziry: Legendary Member; Posts: 869; Joined: Wed Aug 26, 2009 3:49 pmFollowed by: 3 members; Location: California; Thanked: 13 times

Re: Exponents

by heshamelaziry » Mon Sep 21, 2009 6:11 pm

Brent Hanneson wrote:
heshamelaziry wrote:The value of (2^-14 + 2^-15 + 2^-16 + 2^-17)/5 is how many times the value of 2^-17 ?

The answer seems to have a flaw in multiplying fractions !!!
For some reason, I'm having a hard time adding an image to the post. So, I've attached the solution.

Cheers,
Brent

it shuold be ((2^-14 + 2^-15 + 2^-16 + 2^-17)/5 )/ 2^-17 which translates to:

((2^-14 + 2^-15 + 2^-16 + 2^-17)/5 * 1/ 2^-17.

I don't know what i am missing ! i.e (2/5)/4 = 2/5 * 1/4. Shouldn't it be the same principal ?

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GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

Re: Exponents

by Brent@GMATPrepNow » Mon Sep 21, 2009 6:19 pm

heshamelaziry wrote:
Brent Hanneson wrote:
heshamelaziry wrote:The value of (2^-14 + 2^-15 + 2^-16 + 2^-17)/5 is how many times the value of 2^-17 ?

The answer seems to have a flaw in multiplying fractions !!!
For some reason, I'm having a hard time adding an image to the post. So, I've attached the solution.

Cheers,
Brent
it shuold be ((2^-14 + 2^-15 + 2^-16 + 2^-17)/5 )/ 2^-17 which translates to:

((2^-14 + 2^-15 + 2^-16 + 2^-17)/5 * 1/ 2^-17.

I don't know what i am missing ! i.e (2/5)/4 = 2/5 * 1/4. Shouldn't it be the same principal ?

It's probably easier to rewrite ((2^-14 + 2^-15 + 2^-16 + 2^-17)/5 )/ 2^-17 as ((2^-14 + 2^-15 + 2^-16 + 2^-17)/5 ) x 2^17

Then you get: (2^3 + 2^2 + 2^1 + 1)/5 = 3

Useful rule here: 1/(x^-n) = x^n

Brent Hanneson - Creator of GMATPrepNow.com

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