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If a light bulb is selected at random from a shipment, what

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If a light bulb is selected at random from a shipment, what

by AAPL » Sat Mar 16, 2019 6:05 am

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If a light bulb is selected at random from a shipment, what is the probability that the light bulb is defective?

1) The ratio of the number of defective light bulbs to the number of nondefective light bulbs is 1 to 60.
2) The shipment contains 720 light bulbs.

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by Ian Stewart » Sun Mar 17, 2019 11:39 am

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When you're only selecting one thing from a group, a probability is just a ratio. If you're asked, say, "if you pick a random student from a class, what is the probability you pick a woman?", that is the same question as "what fraction of the students are women?" So the question here is just asking "what fraction of the bulbs are defective?" Statement 1 clearly tells us 1/61 of the bulbs are, while Statement 2 is useless.

Note also that there's a problem with the question -- the two statements can't both be true. If the ratio of defective to nondefective bulbs is 1 to 60, then the total number of bulbs absolutely must be a multiple of 61, and 720 is not a multiple of 61.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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