Simple Bacteria doubling problem

[Neil6790]

Homework Statement

The doubling period of a baterial population is 20 minutes. At time t = 120 minutes, the baterial population was 80000. With t representing minutes, the formula for the population is p(t)=A e^{kt}.

k=?
The initial population at time t = 0 is:?
The size of the baterial population after 3 hours is: ?



I attempted this problem and came up with k being (ln(2))/140. The online assignment keeps saying that is the wrong answer. If i can't find k, I can't get anything else. Please help me out.

 

[Dick]

If the doubling time is twenty minutes then the population at time t=20 is twice the population at t=0. So 2*A*e^(k*0)=A*e^(k*20). Can you solve that for k? Notice the A cancels.

 

[Neil6790]

I am not sure. I can't solve that for k. I don't have A.

 

[Dick]

A cancels!

 

[Neil6790]

oh wow silly me. sorry. it does cancel

 

[Neil6790]

i got (ln(2))/20. Now how do i find t=0 if i don't know what A is.

 

[Dick]

Good start! You know 80000=A*e^(k*120). Can't you find A from that?