Dr. Johnson is four times as old as his son. In twenty years, he’ll be twice as old.

How old are they now?

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Answer:

Dr. Johnson is 40, his son is 10.

Let y be Dr. Johnson’s age and x be his son’s age, then y=4x and y+20=2(x+20). Solving this system of two equations we get x=10 and y=40.

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