Question

A culture is grown with a population of 45 million cells. The cell population is found to double every hour. What will the population be after four hours?

Answer 1

45 million * $2^{4}$ = 45million * 16 = 720million

Or you can use 
$y=Ae^{kt}$
A is equal to 45 million
To find the rate, k, we can plug in:
$90million=45million*e^{k*1}$
$2=e^{k}$
$k=ln(2)$
$y=4.5*10^{7}*e^{t *ln2}$
Knowing that t = 4
$y = 4.5*10^{7}*e^{ln(2)*4}$
$y = 720 million$

Answer 2

The population will be 360 million people after four hours