Question

There are initially 1000 bacteria in a culture. The number of bacteria doubles each hour. The number of bacteria N present after t hours is . How long will it take the culture to increase to 50,000 bacteria?

Answer 1

There are several information's of immense importance already given in the question. Based on those information's the answer can be easily deduced. 
Initial number of bacteria in the culture = 1000
Number of bacteria after t hours = 1000 (2)^t
Then
1000 (2)^t = 50000
2 ^t = 50
t = 5.6

I hope the procedure is clear enough for you to understand.