so dad is represented by x
son is represented by y
so x is (x=) 7 times his son (7y)
our first sentence is x=7y
10 years later (x+10, y+10) he will be 3 times as old as his son (x+10=3(y+10))
or 10+x=3(y+10)
so our sentences are
x=7y
and
x+10=3(y+10)
first we subsitute x=7y for x in the second equation
(7y)+10=3(y+10)
7y+10=3(y+10)
we distribute
7y+10=3y+30
subtract 3y from both sides
4y+10=30
subtract 10 from both sides
4y=20
divide both sides by 4
y=5
the son is currently 5 years old
x=7y
subsitute y=5 for y in equation
x=7(5)
x=35
the dad is currently 35 years old
in 10 years
dad is 45 and son is 15
dad=35
son=5