Answer: John is now 18 years old
Step-by-step explanation:
Let, John's age is now x years.
5 years ago, he was (x - 5) years old
In 8 years, he will be (x + 8) years old
By the given condition,
x - 5 = 1/2 (x + 8)
⇒ 2 (x - 5) = x + 8
⇒ 2x - 10 = x + 8
⇒ 2x - x = 8 + 10
⇒ x = 18