jmc

Let $t$ and $u$ be the tens and units digits of John's age. We are told that $t+u=5$ and $10u+t-(10t+u)=27$. Rewriting the second equation as $9(u-t)=27$ and dividing by 9, we have $$\begin{array}{rl}t+u& =5\\ t-u& =3.\end{array}$$ Summing these equations, we find $2t=8$, which implies $t=4$. Substituting back into either equation, we find $u=1$, so John's father is $10t+u=\boxed{41}$ years old.