Hello!
The answer is: 23.77 hours
Why?

Where:
Total(t) is equal to the amount for a determined time (in hours)
Start is the original amount
t is the time in hours.
For example, it's known from the statement that the bacteria double their population every 15 hours, so it can be written like this:

To calculate how long it takes for the bacteria cells to increase to 300, we should do the following calculation:

So, to know if we are right, let's replace 23.77 h in the equation:
Total(t)=100*2^\frac{23.77}{15}=299.94

and 299.94≅300
Have a nice day!