If an amount of money, P, the principal, is invested for a period of t months at a monthly interest rate r, the amount of interest, I, earned is:
I = Prt
In this case, we are given a rate of interest per year, then we must divide it by 12 (12 months equivalent to a year) so we can get the monthly rate, then we get:
By replacing 1300 for P, 0.105 (10.5%) for r and 8 for t, we get:
Then, the simple interest earned after 8 months will be $91