The sum of the ages of Mr Lin and his son Ethan is 38 years. In three years' time, Mr Lin will be three times as old as his son. Find their present ages.
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Mr.Lin's present age is 30 years now. His son is 8 years old now.

Step-by-step explanation:

Let L = Mr.Lin's present age,

   S = his son's present age.

First equation  is

L + S = 38.       (1)

The second equation is  

L + 3 = 3*(S+3)   (2)   ("In three years' time, Mr.Lin will be three times as old as his son.")

From (1), express S = 38-L and substitute it into (2). You  will get

L + 3 = 3*((38-L)+3),   or

L + 3 = 114 - 3L + 9,

4L = 114 + 9 - 3,  

4L = 120,

L = 30.