Answer:
268,000
Step-by-step explanation:
You want to know the population of bacteria an hour ago if it was 1 million 20 hours ago and 1/2 million 10 hours ago, and decreasing exponentially.
Exponential function
The population can be described by a function of the form ...
P = p0·(1/2)^(t/h)
where p0 is the initial population, and h is the half-life.
Application
Here, we have p0 = 1 million, h = 10 hours, and we want to evaluate this function at t=19.
The population is ...
P = 1·(1/2)^(19/10) ≈ 0.2679 . . . . million
The population an hour ago was about 268 thousand.
__
Additional comment
The population figures are given to 1 significant figure, so we cannot legitimately report population down to the units. Even the number 268,000 probably has more precision than is justified.
