Cube Root Question

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deagez: Senior | Next Rank: 100 Posts; Posts: 40; Joined: Sun Dec 28, 2008 2:52 am; Location: Denver; Thanked: 2 times

by deagez » Sat Apr 04, 2009 2:50 am

Question is

[(.000064)^1/3]^1/2

A) .0004
B) .008
C) .02
D) .04
E) .2

I mean with guess and check it is easy to figure this out in less than 30 seconds, but just out of curiosity can anyone think of a quick way to do the cube root of .000064 if we didn't have choices?

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Source: Beat The GMAT — Problem Solving | Sat Apr 04, 2009 2:50 am

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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

Re: Cube Root Question

by Ian Stewart » Sat Apr 04, 2009 4:39 am

You can convert decimals to fractions:

[(.000064)^1/3]^1/2 = [(64/10^6)^1/3]^1/2 = (4/10^2)^1/2 = 2/10

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

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cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Sat Apr 04, 2009 4:51 pm

but just out of curiosity can anyone think of a quick way to do the cube root of .000064 if we didn't have choices?

Cube root of 64 is 4

We have six digits after decimal so it will be .04

.2 * .2 = .04

Hence .2

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