precal
posted by Anonymous .
A petri dish contains only two bacteria. The bacteria divide every half hour. How many bacteria will exist after 3 hours?
I thought it'd be 12, but apparently not.

1st hour: 2, 4
2nd hour: 8, 16
3rd hour: 32, 64 
It divides 2 times every hour, so for 3 hours it'll divide 6 times. You start with 2, and every half hour they divide into 2. So that means they divide into 2 a total of 6 times. Soo, that's the original 2 times 2^6. So its 2*64 = 128

A bacteria culture divide each half hour, after 3 hours there are
6400 bacteria, how many bacteria were there to begin with ?