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lolo: Junior | Next Rank: 30 Posts; Posts: 17; Joined: Sat Dec 13, 2008 9:00 am

by lolo » Sat Mar 14, 2009 11:12 am

Jerry's average score on the first three of four tests is 85. If Jerry wants to raise his average by 2 points, what score must he earn on the fourth test ?

87
88
89
91
93.

Can somebody please help me solving this intermediate question ?

I'll give the answer later.

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Source: Beat The GMAT — Problem Solving | Sat Mar 14, 2009 11:12 am

DanaJ: Site Admin; Posts: 2567; Joined: Thu Jan 01, 2009 10:05 amFollowed by: 550 members; Thanked: 712 times; GMAT Score:770

by DanaJ » Sat Mar 14, 2009 11:21 am

The average for the first 3 tests is 85, meaning that t1 + t2 + t3 = 3*85.
In order to raise the average by two points, then the average for the four tests must be 87, or t1 + t2 + t3 + t4 = 4*87.
Notice that t4 will be 4*87 - 3*85 = 4*(85 + 2) - 3*85 = 4*85 + 8 - 3*85 = 4*85 - 3*85 + 8 = 85 + 8 = 93.

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Jose Ferreira: GMAT Instructor; Posts: 99; Joined: Thu Mar 05, 2009 5:14 pmFollowed by: 9 members; Location: NYC; Thanked: 43 times; GMAT Score:800

by Jose Ferreira » Sat Mar 14, 2009 11:30 am

Hi lolo,

Dana is right on. A related way to think about this:

On the fourth test, let's say the score is 85. Then the average across the four tests is 85.

But we want the average score across the four tests to go up by 2 points. So the score on the 4th test has to pick up an additional 2 points for each of the 4 tests. That is, it needs to be 4*2 = 8 points higher. This way, the average score bump per test is 8/4 = 2.

This means that the 4th score must be 85 + 8 = 93.

Jose Ferreira
Founder and CEO, Knewton, Inc.
https://www.knewton.com/gmat

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lolo: Junior | Next Rank: 30 Posts; Posts: 17; Joined: Sat Dec 13, 2008 9:00 am

by lolo » Sat Mar 14, 2009 11:34 am

OK, it's clear for me now.

Thank's.

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franciskyle: Senior | Next Rank: 100 Posts; Posts: 32; Joined: Wed Mar 11, 2009 9:56 pm; Location: Whitler, BC, Canada

by franciskyle » Sat Mar 14, 2009 6:22 pm

You could immediately draw the below equation from the problem - X being the fourth test score.

(3*85 + X)/4 = 87

Therefore X = 93

k. Francis

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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 Mar 14, 2009 6:33 pm

Needs to make up 2 on each of these 3 tests and then score a 87 on the 4th one

87+6 = 93

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