Answer:
....x = the age of the son
y = father's age
The sum of three times the age of the son and the age of his father is 110=
3x+y=110
If three times his father's age is added to the son's age, we get 170 =
x+3y=170
system :
3x+y=110
x+3y=170
by substitution: y=110-3x
we replace y in the other equation:
x+3(110-3x)=170
x+330-9x=170
-8x=170-330
-8x=-160
x=20
the son is 20 years old
we calculate y:
x+3y=170
20+3y=170
3y=170-20
3y=150
y= 50
the father is 50 years old
sum of their ages: 20+50=70