LulaBrazilia wrote:Four hours from now, the population of a colony of bacteria will reach 1.28 x 10^6. If the population of the colony doubles every 4 hours, what was the population 12 hours ago?
A) 6.4 x 10^2
B) 8.0 x 10^4
C) 1.6 x 10^5
D) 3.2 x 10^5
E) 8.0 x 10^6
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
Cheers,
Brent
