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[email protected]: Master | Next Rank: 500 Posts; Posts: 429; Joined: Wed Sep 19, 2012 11:38 pmFollowed by: 4 members; Thanked: 6 times

by [email protected] » Wed Jun 18, 2014 10:04 pm

Please help me understand this average speed problem:

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Source: Beat The GMAT — Problem Solving | Wed Jun 18, 2014 10:04 pm

sanju09: GMAT Instructor; Posts: 3650; Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members; Location: India; Thanked: 267 times; GMAT Score:760

by sanju09 » Wed Jun 18, 2014 10:51 pm

[email protected] wrote:Please help me understand this average speed problem:

320 miles were run @80 mph, this took him 4 hours. 200 percent longer than 4 hours is 4 + 8 = 12 hours, hence the average speed in the entire trip

= total distance/total time

= 640/(4 + 12) = 640/16 = [spoiler]40 mph

Take C[/spoiler]

The mind is everything. What you think you become. -Lord Buddha

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[email protected]: Master | Next Rank: 500 Posts; Posts: 429; Joined: Wed Sep 19, 2012 11:38 pmFollowed by: 4 members; Thanked: 6 times

by [email protected] » Wed Jun 18, 2014 11:05 pm

But where do we use the formula: d(total distance)/(d1/s1+d2/s)? Thanks

sanju09 wrote:
[email protected] wrote:Please help me understand this average speed problem:
320 miles were run @80 mph, this took him 4 hours. 200 percent longer than 4 hours is 4 + 8 = 12 hours, hence the average speed in the entire trip

= total distance/total time

= 640/(4 + 12) = 640/16 = [spoiler]40 mph

Take C[/spoiler]

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confused13: Senior | Next Rank: 100 Posts; Posts: 97; Joined: Tue Dec 17, 2013 3:29 amFollowed by: 2 members; Thanked: 8 times

by confused13 » Thu Jun 19, 2014 12:58 am

can someone explain me why 200% longer is equal to 12h and not 8h?

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GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Thu Jun 19, 2014 1:56 am

Let tâ‚� = the time for the first 320 miles and tâ‚‚ = the time for the second 320 miles.

Since the first half of the trip is traveled at 80mph, tâ‚� = d/r = 320/80 = 4 hours.
Since Mike takes 200% LONGER to travel the remaining 320 miles, tâ‚‚ = tâ‚� + 200% of tâ‚� = 4 + (200/100)(4) = 4 + 8 = 12.
Average speed = (total distance)/(total time) = 640/(4+12) = 40mph.

The correct answer is C.

Last edited by GMATGuruNY on Thu Jun 19, 2014 10:47 am, edited 1 time in total.

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by [email protected] » Thu Jun 19, 2014 10:42 am

Hi confused13,

Many Test Takers mistake a "200% increase" with "doubling" - these two ideas are NOT the same. Here's an example of why:

Let's say you have $10.

If you increase that by 100%, then you have 10 + 10 = $20. So increasing by 100%.... DOUBLES your money

If you increase that by 200%, then you have 10 + 20 = $30. So increasing by 200%....TRIPLES your money

If you increase that by 300%, then you have 10 + 30 = $40. So increasing by 300%....QUADRUPLES your money
Etc.

This prompt mentions 200% longer (time).

We start with 4 hours.

An increase of 100% = 4 + 4 = 8 hours
An increase of 200% = 4 + 8 = 12 hours

Since the two "legs" of the trip take 4 hours and 12 hours, respectively, the total time = 4 + 12 = 16 hours.

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by Brent@GMATPrepNow » Thu Jun 19, 2014 1:23 pm

I'd like to add to Rich's post and say that SOMETIMES 200% does mean "twice" or "doubled."
This occurs when the word "of" is used in conjunction with 200%.

For example, if we say that y is 200% OF x, we are saying that y = 2x. In other words, y equals twice x.
Likewise, if we say that y is 300% OF x, we are saying that y = 3x. In other words, y equals three times x.

Cheers,
Brent

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sanju09: GMAT Instructor; Posts: 3650; Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members; Location: India; Thanked: 267 times; GMAT Score:760

by sanju09 » Sat Jun 21, 2014 11:19 pm

Curious whether [email protected] got it or not :!:

The mind is everything. What you think you become. -Lord Buddha

Sanjeev K Saxena
Quantitative Instructor
The Princeton Review - Manya Abroad
Lucknow-226001

www.manyagroup.com

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