The duration of a perpetual bond is: [(1 + yield) / yield].The duration of a perpetual bond with a yield of 5% is:D5 = 1.05 / .05 = 21 yearsThe duration of a perpetual bond yielding 10 percent is:D10 = 1.10 / .10 = 11 yearsBecause the duration of a zero-coupon bond is equal to its maturity, the 15-year zero-coupon bond has a duration of 15 years.Thus, comparing the 5 percent perpetual bond and the 15-year zero-coupon bond, the 5 percent perpetual bond has the longer duration. Comparing the 10 percent perpetual bond and the 15-year zero, the zero has a longer duration.