What will be Mary’s age when Adam is twice as old as her?
Answer: (4) 15 years
The age of Adam = 20 years
Adam is 4 times as old as Mary.
Then, the age of Mary = 20/4
= 5 years
If the age of Adam is 20 + 10
= 30 years
Then, the age of Mary is 5 + 10 = 15 years.
Thus, Mery will be 15 years old when Adam is twice as old as her.
Hence, the correct answer is option (4) 15 years.