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A told B, “When I was as old as you are now, then your age was four years less than half of my present age". If the sum of the present ages of A and B is 61 years, what is B's present age in years?

Solution

✅ Correct Option: 2

Let AA's present age =a= a years and BB's present age =b= b years


When AA was bb years old, that was (a−b)(a - b) years ago.

At that time, BB's age was b−(a−b)=2b−ab - (a - b) = 2b - a


According to AA's statement, BB's age at that time was a2−4\dfrac{a}{2} - 4

2b−a=a2−42b - a = \dfrac{a}{2} - 4


4b−2a=a−84b - 2a = a - 8

4b=3a−84b = 3a - 8 ... (1)


Sum of present ages is 6161:

a+b=61a + b = 61

a=61−ba = 61 - b ... (2)


Substituting (2) into (1):

4b=3(61−b)−84b = 3(61 - b) - 8

4b=183−3b−84b = 183 - 3b - 8

7b=1757b = 175

b=25b = 25


Therefore a=61−25=36a = 61 - 25 = 36

BB's present age is 2525 years.

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