Question 235938
Let f = father's present age
Let s = son's present age
:
Write an equation for each statement:
:
"In 17 years a father will be twice as old as his son,"
f + 17 = 2(s + 17)
f + 17 = 2s + 34
f = 2s + 34 - 17
f = 2s + 17
:
" whose age at the present time is one third of his father's age."
s = {{{1/3}}}f
or we can say
f = 3s
:
 How old is the father now? 
;
Easy to find s first, replace f with 3s in the 1st equation
3s = 2s + 17
3s - 2s = 17
s = 17 yrs is son's present age
then
3(17) = 51 yrs is father's present
;
;
Check solution in the statement
In 17 years a father will be twice as old as his son,"
51 + 17 = 2(17 + 17)
68 = 2(34)