Question

A mean of age of the father and his son is 27 years.after 18 years father will be twice as old as his son.their present age was?

Answer 1

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Let the father and son's age be represented by x and y respectively
From the 1st sentence
(x+y)/2 = 27
Cross multiply the equation
x+y = 2*27
x+y = 54..........(1)
From the 2nd sentence 
x+18 = 2(y+18)
x+18 = 2y+36
x-2y = 36-18
x-2y = 18..........(2)
Bring the two equations together
x+y = 54..........(3)
x-2y = 18..........(4)
Eliminate equation (4) from equation (3)
3y = 36
y = 36/3
y = 12
Substitute y = 12 in equation (3)
x+y = 54
x+12 = 54
x = 54-12
x = 42
Therefore there present ages =
Father = x = 42
Son = y = 12