Question : Directions: Adam who is 20 years old is 4 times as old as Mary. What will be Mary's age when Adam is twice as old as her?
Option 1: 35 years
Option 2: 15 years
Option 3: 30 years
Option 4: 17 years
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Correct Answer: 15 years
Solution :
Given:
Adam's age = 20 years
Also, Adam's age = 4 × Mary's age
⇒ Mary's age = Adam's age ÷ 4
⇒ Mary's age = 20 ÷ 4 = 5 years.
Let us assume that after A years, Adam's age is twice the Mary's age;
Adam's age after A years = (20 + A) years, and Mary's age after A years = (5 + A) years.
Also, after A years, Adam’s age = 2 × Mary’s age
⇒ (20 + A) = 2(5 + A)
⇒ 20 + A = 10 + 2A
⇒ 20 – 10 = 2A – A
⇒ A = 10 years
Therefore, Mary's age after 10 years will be 15 years, and Adam’s age after 10 years will be 30 years.
So, Mary's age after 10 years will be 15 years. Hence, the second option is correct.
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