Age

let $M$ and $S$ be the present ages of Marvin and his son, respectively

At present Marvin is $50$ years old.

In $10$ years Marvin will be $60$ years old and his son will be $S+10$ years old.

From the problem,

$60=4(S+10)$

Dividing both sides by $4$ , we get

$15=S+10$

Finally,

$S=15-10=$ $\boxed{5}$

this math needs no heavy equation.

after 10 years Marvin will be(50+10) years old or 60 years old.

so ,his son's age will be 60/4=15 years old.

so, now the sons age is  15-10=5 years old