AIM TO CRACK GMAT wrote:The waiter at an expensive restaurant has noticed that 60% of the couples order dessert and coffee. However, 20% of the couples who order dessert don't order coffee. What is the probability that the next couple the waiter seats will not order dessert?
20% 25% 40% 60% 75%
Let total people = 100.
We can plug in the answers, which represent the number of people who do not order dessert.
Answer choice C: 40 do not order dessert.
Thus, the total number who order dessert = 60.
Too small.
It is given that the number who order BOTH dessert and coffee = 60.
Since some people order ONLY dessert but not coffee, the TOTAL number who order dessert must be GREATER than 60.
To INCREASE the total number who order dessert, the number who do not order dessert must DECREASE from 40.
Eliminate C, D and E.
Answer choice B: 25 do not order dessert.
Thus, the total number who order dessert = 75.
Since 20% of these people do not order coffee, the number who order dessert but not coffee = (.2)*75 = 15.
Thus, number who order BOTH dessert and coffee = (total who order dessert) - (number who order dessert but not coffee) = 75-15 = 60.
Success!
The correct answer is
B.
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