Explanation:

Given, the average age of A, B and C is 20 years
⇒ Sum of the ages of A, B and C = 20 x 3 = 60 years
⇒ A + B + C = 60 ..........(1)
Average age of B and C is 25 years
⇒ Sum of the ages of B and C = 25 x 2 = 50 years
⇒ B + C = 50 ..............(2)
From (1) and (2), A = 10 years
Age of A = 10 years
Hence, the correct answer is Option B