Question 529939
Let x=father's present age
Then 80-x=son's present age

Some t years ago, we know that father's age was equal to son's present age,or:
 x-t=80-x collecting like terms we have:
2x-t=80--------------------------------------------------eq1  
At that time, we also know that son's age was (80-x)-t
Now we are told that:
x-t+(80-x)-t=40, simplifying, we get
-2t+80=40
-2t=-40 or
t=20 , substituting t=20 into eq1, we have
2x-20=80
2x=100
x=50 -----------father's present age
80-x=80-50=30-----------son's present age

CK
50+30=80
80=80
and
(50-20)+(30-20)=40
40=40

Hope this helps---ptaylor