This Exam FM sample reference tests Duration and Convexity. A perpetuity-immediate has Macaulay duration 1 + 1/i; duration 17.6 implies i = 1/16.6, and at rate 2i the duration becomes 1 + 16.6/2 = 9.3, choice B.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AChoice A is inconsistent with the closed-form perpetuity-immediate duration identity; no distinct standard single-step error producing it is identifiable.
CChoice C is inconsistent with the closed-form perpetuity-immediate duration identity; no distinct standard single-step error producing it is identifiable.
DChoice D is inconsistent with the closed-form perpetuity-immediate duration identity; no distinct standard single-step error producing it is identifiable.
EChoice E is inconsistent with the closed-form perpetuity-immediate duration identity; no distinct standard single-step error producing it is identifiable.
Original practice · fully worked
Original variant: sensitivity of a perpetual public-radio grant
A perpetual public-radio grant pays at each year-end and has Macaulay duration 21 years at yield i. If the yield becomes 1.5i, calculate the new Macaulay duration.
A 13.00
B 13.67
C 14.33
D 15.00
E 16.00
Variant answer in brief
Duration 21 implies i = 5%; at 7.5%, perpetuity duration is 14.33, choice C.
Setup
Setup
Use the end-of-year perpetuity duration formula to recover the original yield.
21=1+i1⟹i=0.05
Model
Model
The duration of 21 means the reciprocal-yield component is 20, hence i is 5%.
1.5i=0.075
Compute
Compute
At one and a half times that rate, the new yield is 7.5% and duration is 14.3333.
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