Hello!
The answer is: After 60 hours the population of bacteria will be 504
Why?
We can calculate the total population using the following equation:

Where:
Total(t) is equal to the total population
Start is equal to the start population
T is equal to the time in hours
For example, from the statement we know that the population of bacterias doubles every hours, so, using the formula we have:
It proves that every 18 hours, the population of bacterias doubles it size.
We are asked to calculate the popualation after 60 hours, so:

So, after 60 hours the population of bacteria will be 504.
Have a nice day!