A culture of bacteria had a population of 8,000 at 10 AM. The population grew at a fixed percentage rate per hour and reached 64,000 at 7 PM on the same day. If the population continues to grow at that same rate, at what time will the population reach 256,000?

Answer Choices

Correct answer marked below

9 PM of the same day

10 PM of the same day

12 AM of the next day

12:30 AM of the next day

1 AM of the next day

Correct

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Official Explanation

Official

In 9 hours, from 10 AM to 7 PM, the population grew by a factor of 8 (64,000/8,000 = 8). Since the rate of growth is constant, the population doubles every 3 hours:

2 \times 2 \times 2 = 8

over

3 + 3 + 3 = 9

hours.

The same rate of increase (

\times 2

) in the same time period (3 hours). Therefore, to grow 4 times (from 64,000 to 256,000), the population will need an additional 6 hours (3 hours + 3 hours). Hence, the population will reach 256,000 at 7 PM + 6 hours = 1 AM of the next day.

Alternatively, here is the algebraic approach.…

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What this question tests

What it tests

Your ability to apply GMAT math concepts to solve a multi-step problem quickly and accurately. At the 705-805 level, accuracy and speed both matter.

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Overcomplicating the setup — most GMAT problems reward a direct, organized approach over brute calculation.