500 ps

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dunkin77: Master | Next Rank: 500 Posts; Posts: 269; Joined: Sun Apr 01, 2007 5:41 am

by dunkin77 » Wed Apr 04, 2007 11:13 am

Hi,

The correct answer is B) ..can anyone explain? thx!

One-fifth of the light switches produced by a certain factory are defective. Four-fifths of the defective switches are rejected and of the nondefective switches are rejected by mistake. If all the switches not rejected are sold, what percent of the switches sold by the factory are defective?
(A) 4%
(B) 5%
(C) 6.25%
(D) 11%
(E) 16%

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Source: Beat The GMAT — Problem Solving | Wed Apr 04, 2007 11:13 am

gabriel: Legendary Member; Posts: 986; Joined: Wed Dec 20, 2006 11:07 amFollowed by: 1 member; Location: India; Thanked: 51 times

Re: 500 ps

by gabriel » Wed Apr 04, 2007 12:53 pm

dunkin77 wrote:Hi,

The correct answer is B) ..can anyone explain? thx!

One-fifth of the light switches produced by a certain factory are defective. Four-fifths of the defective switches are rejected and of the nondefective switches are rejected by mistake. If all the switches not rejected are sold, what percent of the switches sold by the factory are defective?
(A) 4%
(B) 5%
(C) 6.25%
(D) 11%
(E) 16%

the darkened sentence is incomplete ... chk it out ...

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dunkin77: Master | Next Rank: 500 Posts; Posts: 269; Joined: Sun Apr 01, 2007 5:41 am

by dunkin77 » Wed Apr 04, 2007 1:23 pm

Oops - sorry! here you go - thanks!

One-fifth of the light switches produced by a certain factory are defective. Four-fifths of the defective switches are rejected and 1/20 of the nondefective switches are rejected by mistake. If all the switches not rejected are sold, what percent of the switches sold by the factory are defective?

A) 4%
(B) 5%
(C) 6.25%
(D) 11%
(E) 16%

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Tame the CAT: Senior | Next Rank: 100 Posts; Posts: 46; Joined: Sun Mar 04, 2007 6:27 am

by Tame the CAT » Wed Apr 04, 2007 7:30 pm

I did not get the same answer, I got 4%.

For my setup, please look at the attachment. I thought I had a good grasp of this. Am I missing something?

Attachments

Overlapping Table.xls: (14.5 KiB) Downloaded 154 times

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Cybermusings: Legendary Member; Posts: 559; Joined: Tue Mar 27, 2007 1:29 amFollowed by: 2 members; Thanked: 5 times

by Cybermusings » Thu Apr 05, 2007 1:18 am

Let total number of switches produced be y. Therefore y/5 are defective. 4/5 of y/5 = 4y/25 are rejected.

y/5 - 4y/25 = y/25 of switches which are defective (but not rejected) are sold.

Total number of non-defective switches = 4y/5. Ones rejected = 1/20 of 4y/5 = 4y/100 = y/25 are rejected

Total rejected = 4y/25 + y/25 = 5y/25 = y/5. Therefore total sold = 4y/5

Therefore percent of the switches sold by the factory that are defective =( (y/25) / (4y/5) ) * 100

= 5%

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