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

##### This topic has expert replies
Moderator
Posts: 2064
Joined: 29 Oct 2017
Followed by:2 members

### 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

## Global Stats

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

### GMAT/MBA Expert

GMAT Instructor
Posts: 16086
Joined: 08 Dec 2008
Location: Vancouver, BC
Thanked: 5254 times
Followed by:1267 members
GMAT Score:770

### 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 [email protected] » 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)