Independent solution

How to solve this Perpetuity Duration question

Setup

Setup

For a unit perpetuity-immediate, price is the sum of vᵗ and the time-weighted numerator is the sum of tvᵗ.

P=t1vt=1/iP=\sum_{t\ge1}v^t=1/i

Model

Model

Dividing the two convergent geometric-series identities gives Macaulay duration 1/d, where d = i/(1 + i).

DM=t1tvtt1vt=1dD_M=\frac{\sum_{t\ge1}tv^t}{\sum_{t\ge1}v^t}=\frac1d

Compute

Compute

At i = 10%, 1/d = 1.10/0.10 = 11 years.

DM=1.100.10=11D_M=\frac{1.10}{0.10}=11

Answer

Answer

The calculation gives 11 years for perpetuity duration, matching published choice C.

DM=11(C)\boxed{D_M=11\quad\text{(C)}}