Original practice · fully worked
Original variant: number of deposits supporting a perpetuity
A saver deposits 10 at each year-end into an account earning 4% effective. Immediately after the last deposit, the balance buys a perpetuity-immediate priced at 5% and paying 14.8890 annually. How many deposits were made?
- A
16 - B
18 - C
20 - D
22 - E
25
Answer and calculation
The perpetuity costs 297.7800. Solving 10s-angle-n at 4% equal to that amount gives n = 20, choice C.
Setup
Setup
Translate the perpetuity payment into the fund required at purchase.
X=0.0514.8890=297.780 Model
Model
The year-end deposits accumulate as an annuity-immediate.
10sn∣0.04=297.780 Compute
Compute
Isolate the accumulation factor and solve its exponent.
(1.04)n=1+0.0410297.780=2.19112 Answer
Answer
The saver made 20 deposits, choice C.
n=20(C)