Answer
(a) $T(t)=60+38.6e^{-0.1947t}$
(b) $6$ hours
Work Step by Step
(a) Based on the Newton's Law of Cooling $T(t)=T_s+D_0e^{-kt}$ with the given conditions $T_s=60, D+0=98.6-60=38.6,k=0.1947$ we have $T(t)=60+38.6e^{-0.1947t}$
(b) Given $T=72$, we have $72=60+38.6e^{-0.1947t}$ which gives $t=-ln(\frac{72-60}{38.6})/0.1947\approx6$ hours
