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sqrt(80) +

This topic has 2 expert replies and 0 member replies
boomgoesthegmat Senior | Next Rank: 100 Posts Default Avatar
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sqrt(80) +

Post Wed May 04, 2016 4:29 am
sqrt(80) + sqrt(125) =

A) 9sqrt(5)
B) 20sqrt(5)
C) 41sqrt(5)
D) sqrt(205)
E) 100

OA: A

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GMAT/MBA Expert

Post Wed May 04, 2016 5:51 am
boomgoesthegmat wrote:
√80 + √125 =

A) 9sqrt(5)
B) 20sqrt(5)
C) 41sqrt(5)
D) sqrt(205)
E) 100

OA: A
Another approach is to ESTIMATE

We know that √81 = 9, so √80 is a little bit less than 9
We know that √121 = 11, so √125 is a little bit more than 11

So, √80 + √125 ≈ 9 + 11 ≈ 20

When we check the answer choices, we must keep in mind that √5 ≈ 2.2

A) 9√5 ≈ (9)(2.2) ≈ 20 NICE!!
B) 20√5 ≈ (20)(2.2) ≈ 44 NO GOOD. ELIMINATE
C) 41√5 ≈ (20)(2.2) ≈ TOO BIG. ELIMINATE
D) √205. We know that √225 = 15. So, √205 is too small. ELIMINATE
E) 100 ELIMINATE

Answer: B

Cheers,
Brent

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GMAT/MBA Expert

Post Wed May 04, 2016 5:46 am
boomgoesthegmat wrote:
√80 + √125 =

A) 9sqrt(5)
B) 20sqrt(5)
C) 41sqrt(5)
D) sqrt(205)
E) 100

OA: A
IMPORTANT RULE: √(xy) = (√x)(√y)

So, √80 + √125 = √(16 x 5) + √(25 x 5)
= (√16)(√5) + (√25)(√5)
= 4√5 + 5√5
= 9√5
= A

Related Resource
The following video covers the concepts/strategies that are useful for answering this question:
- Simplifying roots: https://www.gmatprepnow.com/module/gmat-powers-and-roots/video/1038


Cheers,
Brent

_________________
Brent Hanneson – Founder of GMATPrepNow.com
Use our video course along with Beat The GMAT's free 60-Day Study Guide

Check out the online reviews of our course
Come see all of our free resources

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