A pension fund manager seeks to measure the sensitivity of her pension liabilities to market interest rate changes. The manager determines the present value of the liabilities under three interest rate scenarios: a base rate of $7\%,$ a $100$ basis point increase in rates up to $8\%,$ and a $100$ basis point drop in rates down to $6\%.$ The results of the manager$$s analysis are presented below:

Interest Rate Assumption Present Value of Liabilities

$6\%$ $$510\mathrm{.}1$ million

$7\%$ $$455\mathrm{.}4$ million

$8\%$ $$373\mathrm{.}6$ million

The effective duration of the pension fund$$s liabilities is closest to$\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$I just need help understanding why this answer is $14\mathrm{.}99.$ I understand how to calculate it $"510\mathrm{.}1-373\mathrm{.}6*\mathrm{.}0100*455\mathrm{.}4."$ How does this question come out to $14\mathrm{.}99?$