Find the present age of each

Jonathan is now three times as old as his son is. Twelve years ago, he was six times as old as his son was. Find the present age of each.

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x = son's age now
3x = Jonathan's age now   {Jonathan is three times as old as his son}

x - 12 = son's age 12 years ago   {subtracted 12 from current age}
3x - 12 = Jonathan's age 12 years ago   {subtracted 12 from current age}

3x - 12 = 6(x - 12)   {twelve years ago, Jonathan was six times his son's age}
3x - 12 = 6x - 72   {used distributive property}
-3x = -60   {subtracted 6x and added 12 to each side}
x = 20   {divided each side by -3}
3x = 60   {substituted 20, in for x, into 3x}

Jonathan is 60 
his son is 20

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