You start with 23 billion yeast cells (23x10e9) and they have a doubling time of 90 minutes, after how long will your starter hold 90 billion yeast?

Question

You start with 23 billion yeast cells (23x10e9) and they have a doubling time of 90 minutes, after how long will your starter hold 90 billion yeast? 
Please input your answer in minutes

x(t) = x0 × (1 + r) t 
My answer is 136..

is this correct?
Thanks

R_scott · Answer

check your calculations 90 = 23 * 2^(m / 90) log(90 / 23) = (m / 90) log(2)

Ana · Answer

ok Sir would be 270 minutes