Bond A pays $$12,000$ in $40$ years. Bond B pays $$12,000$ in $20$ years. $($To keep things simple, assume these are zero$-$coupon bonds, which means the $$12,000$ is the only payment the bondholder receives.$)$

Suppose the interest rate is $3\mathrm{.}5$ percent.

Using the rule of $70,$ the value of Bond A is approximately$$6,000,$ and the value of Bond B is approximately$$6,000.$

Now suppose the interest rate increases to $7$ percent.

Using the rule of $70,$ the value of Bond A is now approximately $,$ and the value of Bond B is approximately $.$

Comparing each bond's value at $3\mathrm{.}5$ percent versus $7$ percent, Bond A$\text{'}$s value decreases by a percentage than Bond B$\text{'}$s value.

The value of a bond when the interest rate increases, and bonds with a longer time to maturity are sensitive to changes in the interest rate.