For this question, you need to note that the difference in age is always the same, in this case, the difference in Jie Lin and her son's age is 48-12=36 years. Now we use x as her son's age and since Jie Lin will be 3 times as old, her age will be 3x. From here, we can form an equation whereby 3x - x = 36 (note that the age gap is constant). Therefore, 2x = 36, x = 18. We can find Jie Lin's age whereby 3x = 3 x 18 = 54. She will be 3 times as old as her son when she is 54 years old, which is in 54 - 48 = 6 years time. Hope this helps. :)