Put the planets in order based on the length of time it takes them to orbit the sun

250

Explanation:

According to the question the number of bacteria population becomes double in 9 hours to that of 1 hr that means bacteria is dividing in 9 hours. Each time the bacteria divides, the numbers become double.

As we have found that bacteria is dividing in 9 hours so for the first 9 hours the number will be same as initial population i.e. 250. But just after nine hours the population will be double i.e. 500.

So, after 3 hours the population will be same as initial population, 250.