簡單數學一題

Let x be his son's present age.
A man is 28 years older than his son -> A man is x+28 years old now
In 5 years' time he will be five times as old as his son -> A man will be 5(x+5) 5 years later

Therefore
5 years later...
x+28+5 = 5(x+5)
x+33 = 5x + 25
33-25 = 5x-x
8 = 4x
x = 2

His son is 2 years old now, thus a man's present age is 2+28 = 30 years old

Let x be the number of the man present age.

5 ( x - 2 8 + 5 ) = x + 5
5 x - 140 + 25 = x + 5
4 x = 1 2 0
x = 30

The number of the man present age is 30.