370 Berk/DeMarzo, Corporate Finance, Fourth Edition, Global Edition
©2017 Pearson Education, Ltd.
The duration of a security is equal to the weighted-average maturity of its cash flows (Eq. 30.6). Thus,
the duration of a five-year zero coupon bond is five years, and the duration of the nine-year, zero-
coupon bond is nine years (see Ex 30.11).
We cannot determine the durations of the annuities exactly without knowing the current interest rate.
But because the cash flows of an annuity are equal and at regular intervals, the duration of an annuity
must be less than its average maturity (because weighting by present values will put less weight on
later cash flows). Thus, the five-year annuity has a duration of less than (1 + 2 + 3 + 4 + 5) / 5 = 3
years, and the nine-year annuity has a duration of less than (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9) / 9 = 5
years. The ranking is therefore:
five-year annuity, nine-year annuity, five-year zero, nine-year zero.
30-12. You have been hired as a risk manager for Acorn Savings and Loan. Currently, Acorn’s balance
sheet is as follows (in millions of dollars):
When you analyze the duration of loans, you find that the duration of the auto loans is 2.1 years,
while the mortgages have a duration of 7.2 years. Both the cash reserves and the checking and
savings accounts have a zero duration. The CDs have a duration of 1.9 years and the long-term
financing has a 9.2-year duration.
a. What is the duration of Acorn’s equity?
b. Suppose Acorn experiences a rash of mortgage prepayments, reducing the size of the
mortgage portfolio from $151.3 million to $100.9 million, and increasing cash reserves to
$101.7 million. What is the duration of Acorn’s equity now? If interest rates are currently
4% and were to fall to 3%, estimate the approximate change in the value of Acorn’s equity.
(Assume interest rates are APRs based on monthly compounding.)
c. Suppose that after the prepayments in part (b), but before a change in interest rates, Acorn
considers managing its risk by selling mortgages and/or buying 10-year Treasury STRIPS
(zero coupon bonds). How many should the firm buy or sell to eliminate its current interest
rate risk?
a. From Eq. 30.8,