Answer:
11.7 years
Step-by-step explanation:
You want the number of years it takes for an annuity of $40 per month earning 7.5% interest compounded daily to achieve a value of $9000.
The formula for the future value of an ordinary annuity is ...
FV = P((1 +r)^n -1)/(r -1)
where P is the periodic payment, r is the periodic interest rate, and n is the number of periods.
This problem is a bit tricky in that the payment period is 1 month, but the compounding of interest is done daily. That means the effective monthly interest rate (r) is ...
1 +r = (1 +0.075/365)^(365/12) ≈ 1.00626892594
This is the 12th root of the effective annual rate.
Solving for n, we get ...
9000 = 40(1.0063^n -1)/0.0063
0.0063·225 = 1.0063^n -1
log(0.0063·225 +1)/log(1.0063) = n ≈ 140.7885 . . . . . . months
We want the time in years, so we need to divide by 12:
years = 140.7885/12 ≈ 11.7324 ≈ 11.7
It will be about 11.7 years until Abby can afford her vacation.