Bunuel wrote:

A student has answered 7 of the 10 questions on the true-false test correctly and has decided to guess randomly on the rest. If all of the questions are equally weighted, what is the probability of the student receiving a score of 80% or more on the test?

A. 0.125

B. 0.5

C. 0.7

D. 0.8

E. 0.875

In order for the student to get an 80% or higher on the test, he must answer at least 1 of the last 3 questions correctly. So, we can use the following formula:

1 = P(answering at least 1 of last 3 questions correctly) + P(answering none of the last 3 questions correctly)

The probability of answering none of the last 3 questions correctly is 1/2 x 1/2 x 1/2 = 1/8.

Thus, the probability of answering at least 1 of the last 3 questions correctly is 1 - 1/8 = 7/8 = 0.875.

Answer: E

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Jeffery Miller

Head of GMAT Instruction

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