SAT Math Multiple Choice Practice Question 264: Answer and Explanation

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Question: 264

22. A culture of bacteria doubles in population every 2 hours. A sample of 100 bacteria grows to 1,000 bacteria by 4:00 p.m. At what time were there 250 bacteria in this sample?

A. 11:30 am
B. 12 noon
C. 12:30 pm
D. 1:00 pm
E. 2:00 pm

Correct Answer: B


B At 4:00 pm, there are 1,000 bacteria. Since the population doubles every two hours, there must have been half as many two hours ago. So at 2:00 pm there were 500 bacteria, and at 12:00 noon there were 250 bacteria. (Notice that the fact that there were 100 bacteria to start is irrelevant.)

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