Question 1062268
Hi there,
the sum of my parents' ages is 70
Father = x
Mother = y
x + y = 70........Eq(1)
x = 70 - y........Eq(1)
Twenty years ago, my father was 4 years 
older than my mother.  
x - 20 = 4 + (y - 20)........Eq(2)
Replace x in Eq(2) with 70 - y
(70 - y) - 20 = 4 + (y - 20)......Eq(2)
70 - y - 20 = 4 + y - 20
Collect like terms
- y - y = 4 - 20 + 20 - 70
-2y = -66
Multiply both sides by -1
2y = 66
y = 33
Substitute y = 33 into Eq(1)
x + y = 70.........(1)
x + 33 = 70
x = 70 - 33
x = 37
Mother is 33 years old
Father is 37 years old.
Hope this helps :-)