Proof the Following Three Propositions Using a Simple Numerical Example

Proof the following three propositions using a simple numerical example for each of the cases.
a. The duration of an asset or liability with intervening cash flows between issue and maturity is smaller than its maturity.
b. The duration of an asset or liability without any intervening cash flows between issue and maturity equals its maturity.
c. Despite the fact that perpetuities such as consol bonds have no maturity, it is possible to calculate their duration.