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Exponential growth problem
By Anonymous on Friday, March 9, 2001 - 10:02 pm:

A culture of bacteria doubles every 15 min. At time of 1¼h the count is 16 000 bacteria. What was the count at time zero? What will be the count at 2½h?

What I did for this question is I took away half of 16 000 and then half of that answer and so on, until I got to time zero. But I don't know how to get it from y=2x. Can someone help me out. Thankyou

By Olof Sisask (P3033) on Friday, March 9, 2001 - 11:46 pm:

If you let N = A × 2t/15, where t is the time in minutes, and N is the number of bacteria, you should be able to see that when the time is 0, N = A, so A is the number of bacteria present initially.

To work out A, you simply substitute the values for N and t that you are given into the equation (remember to convert the time into minutes if you are using the 2t/15 method).

Then for the second part of the question, you've already worked out A, so you just substitute t in the equation and work out N.

Regards,
Olof.