b
$T - T _{0}=\left( T _{ i }- T _{0}\right) e ^{-\frac{ Bt }{ ms }}$
$6 \lambda=\ln 1.5$
$40=60 \,e ^{-\lambda(6)} \Rightarrow 6 \lambda=\ln 1.5$
$20=60 \,e ^{-\lambda t _{2}} \Rightarrow t _{2} \lambda=\ln 3$
$\frac{ t _{2}}{6}=\frac{\ln 3}{\ln 1.5}$
$\therefore t _{2}=16.25 min$
So $\approx 16$