Answer
(a). $P=7.337$
(b). $t=1.733$
Work Step by Step
$P=\frac{10}{1+4e^{-0.8t}}$
(a). for $t=3,$
$P=\frac{10}{1+4e^{-2.4}},$
$P=7.337$
(b).
$5=\frac{10}{1+4e^{-0.8t}},$
$5(1+4e^{-0.8t})=10,$
$1+4e^{-0.8t}=2,$
$e^{-0.8t}=0.25,$
$-0.8t=\ln {0.25}$
$t=1.733$