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In a box of 12 pens, a total of 3 are defective. If a customer buys 2 pens selected at random from the box, what is the

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In a box of 12 pens, a total of 3 are defective. If a customer buys 2 pens selected at random from the box, what is the

by BTGmoderatorDC » Fri Jul 10, 2020 7:05 pm

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In a box of 12 pens, a total of 3 are defective. If a customer buys 2 pens selected at random from the box, what is the probability that neither pen will be defective?

A. 1/6
B. 2/9
C. 6/11
D. 9/16
E. 3/4


OA C

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Re: In a box of 12 pens, a total of 3 are defective. If a customer buys 2 pens selected at random from the box, what is

by Brent@GMATPrepNow » Sat Jul 11, 2020 9:14 am
BTGmoderatorDC wrote: ↑
Fri Jul 10, 2020 7:05 pm
 In a box of 12 pens, a total of 3 are defective. If a customer buys 2 pens selected at random from the box, what is the probability that neither pen will be defective?

A. 1/6
B. 2/9
C. 6/11
D. 9/16
E. 3/4


OA C

Source: Official Guide
P(neither pen is defective) = P(1st pen selected is NOT defective AND 2nd pen selected is NOT defective)
= P(1st pen selected is NOT defective) x P(2nd pen selected is NOT defective)
= 9/12 x 8/11
= 6/11
= C

ASIDE: How did I get 9/12 and 8/11?
For the first selection, 9 of the 12 pens are good.
For the second selection, we must assume that the first selection resulted in a GOOD pen. This means there are now 11 pens remaining, and 8 of them are GOOD.
Brent Hanneson - Creator of GMATPrepNow.com
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Re: In a box of 12 pens, a total of 3 are defective. If a customer buys 2 pens selected at random from the box, what is

by ygcrowanhand » Wed May 18, 2022 10:36 am
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