Question

a man is four times as old as his son 20 year hence he will be twice as old as his son. find their present age.

Answer 1

Let us take the age of his son as y
Therefore,  the age of the man is 4y

Now, It is given that after 20 years the man will be twice as old as his son
                                           
Therefore,

(4y + 20)  =  2(y + 20)
4y + 20    =  2y + 40
          4y  = 2y + 40 - 20
    4y - 2y = 20
            2y = 20
              y = 10

Therefore the age of the man's son is 10 and the age of the man = 4y = (4)(10) = 40