# SAT Math Multiple Choice Practice Question 409: Answer and Explanation

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**Question: 409**

**13.** A researcher found that the number of bacteria in a certain sample doubles every hour. If there were 6 bacteria in the sample at the start of the experiment, how many bacteria were there after 9 hours?

A. 54

B. 512

C. 1,536

D. 3,072

E. 6,144

**Correct Answer:** D

**Explanation:**

**D** This geometric sequence can be expressed as 6 × 2* ^{x}*, where

*x*is the number of hours. So, after 9 hours, there will be 6 × 2

^{9}= 6 × 512 = 3,072. Alternatively, you could just work out the problem each hour by doubling. So, after the first hour, there are 6 × 2 = 12. Then, after the second hour, there are 12 × 2 = 24, and so on, until you get to the ninth hour.