A man is 4 times as old as his son. In 3 years, the father will be 3 times as old as his son. How old is each of them now?

Let's establish the
followings:

a = the age of the son,
now

b = the age of the father,
now.

a+3 = the age of the son, after 3
years.

b+3 = the age of the father, after 3
years.

Now, the father's age is 4 times the son's
age.

b = 4a  (1)

After 3
years, the father's age will be:

b+3 = 3(a+3) 
(2)

We'll substitute the equation (1) into the equation
(2).

4a+3 = 3a + 9

4a - 3a =
9-3

a = 6

b =
4*6

b = 24

The son is 6 years
old and his father is 27 years old, now.