Bunuel wrote:

What is the probability that Lee will make exactly 5 errors on a certain typing test?

(1) The probability that Lee will make 5 or more errors on the test is 0.27.

(2) The probability that Lee will make 5 or fewer errors on the test is 0.85.

NEW question from GMAT® Quantitative Review 2019

(DS06810)

Let P(A)=Probability that Lee will make 5 or more errors on the test.

P(B)=Probability that Lee will make 5 or fewer errors on the test.

We have, P(A or B)=P(A)+P(B)-P(A and B)------------(1)

Question stem:- Probability that Lee will make exactly 5 errors on a certain typing test=P(A and B)=?

From (1), we have P(A and B)=P(A)+P(B)-P(A or B)---------------(2)

St1:-

The probability that Lee will make 5 or more errors on the test is 0.27.Or, P(A)=0.27.

We can't determine P(A and B).

Insufficient.

St2:-

The probability that Lee will make 5 or more errors on the test is 0.27.Or, P(B)=0.85.

We can't determine P(A and B).

Insufficient.

Combining, we have P(A or B)=1 [Lee makes at least 5 errors or at most 5 errors]

P(A and B)=P(A)+P(B)-P(A or B)

Or, P(A and B)=0.27+0.85-1=1.12-1=0.12

Sufficient.

Ans. (C)

_________________

Regards,

PKN

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