This is a geometric sequence:
First month: 8
Second month: 8 * 1.02
Third month: 8*1.02^2
Fourth month 8* 1.02^3
...
nth month: 8 * 1.02 ^(n-1)
Sum of the first 9 terms (months) = a[1 -r^n] / (1 - r) = 8* [1 - 1.02^9] / (1 - 1.02) = 78.04
Answer: approximately 78 bikes.