Exponential and Logarithmic functions

Question

The doubling period of a bacterial population is 15 minutes. At time t=90 minutes, the bacterial population was 70000.

What was the initial population at time t=0?   

Find the size of the bacterial population after 3 hours.

Answer 1

70000 = a₀(2)^90/15

70000 = a₀(2)⁶

70000 = a₀(64)

109.375 = a₀, this does not make sense. Review post for accuracy.