Four years ago the average age of A, B and C was 25 years. Five years ago the average age of B and C was 20 years. A's present age is

Four years ago average age of A , B & C was 25 years.

A's present age

Let the ages of A , B , C be A , B , C years.

We know that,

Average = Sum of observations/Number of observations.

Here,

Sum of observations = A - 4 + B - 4 + C - 4 = (A + B + C - 12) years.

Number of observations = 3.

So, (A + B + C - 12)/3 = 25

⟹ A + B + C - 12 = 3 * 25

⟹ A + B + C = 75 + 12

⟹ A + B + C = 87 -- equation (1)

Also given that,

Average age of B & C before 5 years was 20 years.

⟹ (B - 5 + C - 5)/2 = 20

⟹ B + C - 10 = 2 * 20

⟹ B + C = 40 + 10

⟹ B + C = 50 -- equation (2)

Subtract equation (2) from equation (1).

⟹ A + B + C - (B + C) = 87 - 50

⟹ A + B + C - B - C = 37

⟹ A = 37 years

∴ A's present age is 37 years.

I hope it will help you.

Regards.

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