A company found out that the rate at which its machine operators' efficiency, F in $(\%),$ changes with respect to the amount of hours $,$ t$,$ that they have been working during the day, follows the model below

$,$ where

a$)$ What is the efficiency function F$,$ if F$(5)=68\mathrm{.}71875$

b$)$ What is the efficiency of the operator after $7$ hours?