Question 522397
x = man's age now
y = son's age now
.
x + y = 39
so
x = 39-y
.
In three years:
x+3 = man's future age
y+3 = son's future age
.
The man's age will be 4 times his son's age:
(x+3) = 4(y+3)
.
Substitute
.
(39-y+3) = 4(y+3)
.
Solve
.
42-y = 4y + 12
30-y = 4y
30 = 5y
5y = 30
y = 6
.
x+y = 39
x + 6 = 39
x = 33
.
Check:  In three years, will the man's age be 4 times the son's age?
x+3 = 36
y+3 = 9
x+3 = 4(y+3)
Correct.
.
Answer:  The man is 33, and the son is 6.
.
Done.