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(1) L + S = 53 ("The sum of the ages of Mr. lee and his son is 53 now")
(2) L-4 = 4*(S-4) ("4 years ago, the age of Mr. Lee was four times that of his son")
From equation (1), express L = 53-S and substitute it into equation (2). You will get
(53-S) - 4 = 4*(S-4)
49 - S = 4S - 16
49 + 16 = 4S + S
65 = 5S ====> S = 65/5 = 13.
Answer. The sun is 13 years old. The father (= Mr. Lee) is 53-13 = 40 years old.
Solved.
