Abacteria culture begins with 8 bacteria which triple in size every week. how many bacteria exist in the culture after 4 weeks?

The given trend for the bacteria population is a geometric sequence with initial amount of 8 (a0) and the common ratio equal to 3. The amount of bacteria at any time t is,
                                          at = (a0) x r^(t - 0)
Substituting the given,
                                           at = (8) x 3^(4 - 0)
The numerical value for at is 648. Thus, there will be 648 bacteria after 4 weeks. 

This is what i did 45-5=4040÷4=10so there are 10 white marbles but am not sure is just a question that am not sure

Reduction in overall biodiversity