Question

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 ​

Answer 1

→ md - 2nd + 3md = 2 * [ a + (n - 1)d ]

→ 4md - 2nd = 2 * [ a + (n - 1)d ]

→ 2(2md - nd) = 2 * (a + nd - d)

→ 2md - nd - a + d = nd

→ 2md - nd - (md - 2nd) + d = nd

[ From equation (1) ]

→ 2md - nd - md + 2nd + d = nd

→ md + nd + d = nd

→ (m + n + 1)d = n * d

→ (m + n + 1) = n

Explanation:

Answer 2

Given:

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

Find:

A's present age

Solution:

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

• Age of A before 4 years = (A - 4) years.
• Age of B before 4 years = (B - 4) years.
• Age of C before 4 years = (C - 4) 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.