## Four hours from now, the population of a colony of bacteria will reach $$1.28*10^6.$$ If the population of the colony

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### Four hours from now, the population of a colony of bacteria will reach $$1.28*10^6.$$ If the population of the colony

by AAPL » Mon Feb 21, 2022 7:10 am

00:00

A

B

C

D

E

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Official Guide

Four hours from now, the population of a colony of bacteria will reach $$1.28*10^6.$$ If the population of the colony doubles every $$4$$ hours, what was the population $$12$$ hours ago?

A. $$6.4*10^2$$
B. $$8.0*10^4$$
C. $$1.6*10^5$$
D. $$3.2*10^5$$
E. $$8.0*10^6$$

OA B

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### Re: Four hours from now, the population of a colony of bacteria will reach $$1.28*10^6.$$ If the population of the colon

by Brent@GMATPrepNow » Mon Feb 21, 2022 10:06 am
AAPL wrote:
Mon Feb 21, 2022 7:10 am
Official Guide

Four hours from now, the population of a colony of bacteria will reach $$1.28*10^6.$$ If the population of the colony doubles every $$4$$ hours, what was the population $$12$$ hours ago?

A. $$6.4*10^2$$
B. $$8.0*10^4$$
C. $$1.6*10^5$$
D. $$3.2*10^5$$
E. $$8.0*10^6$$

OA B
Let's work backwards.

Population 4 hours in future: 1.28 x 10^6
Population now: 0.64 x 10^6 (half the population 4 hours in future)
Population 4 hours ago: 0.32 x 10^6 (half the current population)
Population 8 hours ago: 0.16 x 10^6 (half the population 4 hours ago)
Population 12 hours ago: 0.08 x 10^6 (half the population 8 hours ago)

Now check the answer choices.
Only answer choices B and E have the same format with 8 times some power of 10
Answer choice E definitely doesn't match, so the correct answer must be B

For any doubters out there, notice that:
0.08 x 10^6 = (8.0) x (10^-2) x 10^6
= 8.0 x 10^4

Answer: B
Brent Hanneson - Creator of GMATPrepNow.com

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