32
108. Solution: A
Let x, y, and z represent the amounts invested in the 5-year, 15-year, and 20-year zero-coupon
bonds, respectively. Note that in this problem, one of these three variables is 0.
The present value, Macaulay duration, and Macaulay convexity of the assets are, respectively,
22 2
5 15 20 5 15 20
,,
x y zx y z
xyz
xyz xyz
++ + +
++
++ ++
.
We are given that the present value, Macaulay duration, and Macaulay convexity of the liabilities
are, respectively, 9697, 15.24, and 242.47.
Since present values and Macaulay durations need to match for the assets and liabilities, we have
the two equations
5 15 20
9697, 15.24
xyz
xyz
xyz
++
++= =
++
.
Note that 5 and 15 are both less than the desired Macaulay duration 15.24, so z cannot be zero.
So try either the 5-year and 20-year bonds (i.e. y = 0), or the 15-year and 20-year bonds (i.e. x =
0).
In the former case, substituting y = 0 and solving for x and z yields
(20 15.24)9697
3077.18
20 5
x
−
= =
−
and
(15.24 5)9697
6619.82
20 5
z
−
= =
−
.
We need to check if the Macaulay convexity of the assets exceeds that of the liabilities.
The Macaulay convexity of the assets is
22
5 (3077.18) 20 (6619.82)
281.00
9697
+
=
, which exceeds
the Macaulay convexity of the liabilities, 242.47. The company should invest 3077 for the 5-year
bond and 6620 for the 20-year bond.
Note that setting x = 0 produces y = 9231.54 and z = 465.46 and the convexity is 233.40, which
is less than that of the liabilities.
109. Solution: E
The correct answer is the lowest cost portfolio that provides for $11,000 at the end of year one
and provides for $12,100 at the end of year two. Let H, I, and J represent the face amount of each
purchased bond. The time one payment can be exactly matched with H + 0.12J = 11,000. The
time two payment can be matched with I + 1.12J = 12,100. The cost of the three bonds is H/1.1 +
I/1.2321 + J. This function is to be minimized under the two constraints. Substituting for H and I
gives (11,000 – 0.12J)/1.1 + (12,100 – 1.12J)/1.2321 + J = 19,820 – 0.0181J. This is minimized
by purchasing the largest possible amount of J. This is 12,100/1.12 = 10,803.57. Then, H =
11,000 – 0.12(10,803.57) = 9703.57. The cost of Bond H is 9703.57/1.1 = 8,821.43.