Probability of bulb testing

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pakaskwa: Master | Next Rank: 500 Posts; Posts: 107; Joined: Wed Mar 04, 2009 4:39 am; Location: Vancouver; Thanked: 9 times; GMAT Score:750

by pakaskwa » Fri Mar 06, 2009 1:46 am

If you test 2 different lightbulbs from a box of 100 lightbulbs that contain 1 defective lightbulb, what is the probability that both lightbulbs that you test are defective?
A. 0
B. 0.0001
C. 1/9900
D. 0.01
E. 1/99

OA answer is A. Can you please explain?

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Source: Beat The GMAT — Problem Solving | Fri Mar 06, 2009 1:46 am

kamu: Senior | Next Rank: 100 Posts; Posts: 49; Joined: Fri Feb 27, 2009 12:41 amFollowed by: 3 members; Thanked: 44 times

by kamu » Fri Mar 06, 2009 1:57 am

The question says there is only one defective bulb.,

We cannot possibly find two different defective bulbs!

A is the only possible answer!!!

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pakaskwa: Master | Next Rank: 500 Posts; Posts: 107; Joined: Wed Mar 04, 2009 4:39 am; Location: Vancouver; Thanked: 9 times; GMAT Score:750

by pakaskwa » Fri Mar 06, 2009 2:12 am

My understanding of the question was that they only tested the first 2 bulbs.

Do you think it's a fair question?

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