Winston wants to save $8900 to buy a used car in 2 years. If his savings account earns interest at 7.8%
compounded weekly, how much does he need to deposit each week?
assuming an ordinary annuity, just use the formula
FV = P((1+i)^n - 1)/i)
since your weekly rate is 7.8%/52 = 0.0015, and 2 years is 104 weeks, that would mean you want P such that
P(1.0015^104 - 1)/0.0015 = 8900
so now just crank it out.