Independent solution

How to solve this Annuities question

Setup

Setup

Index the 26 beginning-of-year deposits at times 0 through 25 and use time 0 as the comparison date.

tk=k−1,k=1,…,26t_k=k-1,\qquad k=1,\ldots,26

Model

Model

Deposits before or at time 10 use only the 3% discount curve; later deposits first cross the 2% segment after time 10.

v(t)={(1.03)−t,0≤t≤10,(1.03)−10(1.02)−(t−10),t>10.v(t)=\begin{cases}(1.03)^{-t},&0\le t\le10,\\(1.03)^{-10}(1.02)^{-(t-10)},&t>10.\end{cases}

Compute

Compute

Equate the discounted target balance at time 25 to the two groups of discounted deposits.

100000(1.03)10(1.02)15=X∑k=111(1.03)−(k−1)+X∑k=1226(1.03)−10(1.02)−(k−11)\frac{100000}{(1.03)^{10}(1.02)^{15}}=X\sum_{k=1}^{11}(1.03)^{-(k-1)}+X\sum_{k=12}^{26}(1.03)^{-10}(1.02)^{-(k-11)}

Answer

Answer

This is exactly the equation displayed in choice D.

choice D\boxed{\text{choice D}}