Abacteria culture begins with 15 bacteria which double in amount at the end of every hour. how many bacteria are grown during the 12th hour?

The answer is 30720

A = P * rⁿ
A - the final population
r - the growth rate
P - the initial number of bacteria
n - the number of hours

We know:
A = ?
r = 2 (it doubles)
P = 15
n = 12

After 11 hours
A(11) = 15 * 2¹²
A(11) = 15 * 2048
A(11) = 30720

After 12 hours
A(12) = 15 * 2¹²
A(12) = 15 * 4096
A(12) = 61440

How many bacteria are grown during the 12th hour?
A(12) - A(11) = 61440 - 30720 = 30720

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