M Prep F of Math pp 104 Exponent Question 75^3*45^3/15^8

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mkofron: Newbie | Next Rank: 10 Posts; Posts: 5; Joined: Wed Oct 26, 2016 8:44 am

by mkofron » Mon Oct 31, 2016 2:58 pm

75^3*45^3/15^8

The answer is 15.

Why can't you just reduce everything by 15 and go from there? I tried this an it didn't work. I just want to know why I can't do this.

Thank you for the help.

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Source: Beat The GMAT — Problem Solving | Mon Oct 31, 2016 2:58 pm

melguy: Master | Next Rank: 500 Posts; Posts: 335; Joined: Mon Mar 21, 2011 11:31 pmFollowed by: 2 members; Location: Australia / India; Thanked: 37 times

by melguy » Mon Oct 31, 2016 5:54 pm

75Â³ x 45Â³
---------
15^8

75Â³ x 45Â³
-----------------
15Â³ x 15Â³ x 15Â² (I have broken it down just to show you easier division)

Solve and you get

15Â³
---- = 15
15Â²

P.S. You can multiply and divide same powers e.g. 3Â³ x 15Â³ = (3 x 15)Â³.
Incorrect 3Â² x 15Â³ = (3 x 15)Â²+Â³

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[email protected]: Elite Legendary Member; Posts: 10392; Joined: Sun Jun 23, 2013 6:38 pmFollowed by: 511 members; Location: Palo Alto, CA; Thanked: 2867 times; GMAT Score:800

by [email protected] » Tue Nov 01, 2016 9:30 am

Hi mkofron,

When dealing with complex-looking calculations, it sometimes helps to create a much-simpler example, so that you can be clear on how the rules of math apply.

Consider this simple example:

(4)(8)/2

What does the above fraction equal? It doesn't take much work to calculate it:

(4)(8)/2 = 32/2 = 16

Now what would have happened if you just "reduced everything by 2?".... You would have ended up with 8, which is NOT correct. Thus, you cannot just 'reduce everything by 15" in this question.

GMAT assassins aren't born, they're made,
Rich

Contact Rich at [email protected]

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Matt@VeritasPrep: GMAT Instructor; Posts: 2630; Joined: Wed Sep 12, 2012 3:32 pmFollowed by: 119 members; Location: East Bay all the way; Thanked: 625 times; GMAT Score:780

by Matt@VeritasPrep » Fri Nov 11, 2016 4:25 pm

You actually can!

75Â³ * 45Â³ / 15â�¸ =

15Â³ * 5Â³ * 15Â³ * 3Â³ / (15Â³ * 15Â³ * 15Â²) =

5Â³ * 3Â³ / 15Â² =

15

Save $100 on any VP online course!

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