Question 201804
Let son’s age now be s
Then, 10 years ago, son’s age was s – 10

Therefore, father’s age, 10 years ago was 2(s – 10) + 35 = 2s – 20 +35 = 2s + 15, which means that father’s age now is 2s + 15 + 10 = 2s + 25


To find the amount of years for father’s age to be twice his son’s, and using y as those amount of years, we get:  2(s + y) = 2s + 25 + y

2s + 2y = 2s + 25 + y

y = 25


So, in {{{highlight(25  years)}}}, father will be twice his son’s age


To find the amount of years for father’s age to be thrice his son’s, and using y as those amount of years, we get:  3(s + y) = 2s + 25 + y

3s + 3y = 2s + 25 + y

2y = - s + 25


{{{highlight(y  =  (-s + 25)/2)}}}


So, in {{{highlight((-s + 25)/2)}}}, father will be/was thrice his son’s age, where "s" represents the son's age, today.