Question

Five years ago, the average age of p and q was 15 years. now, average age of p, q and r is 20 years. what would be the age of r after 10 years

Answer 1

let the present age of p and q are x and y years respectively
$\frac{x - 5 + y - 5}{2} = 15 \\ x + y - 10 = 30 \\ x + y = 40 \: \: \: \: \: eq1$
let the present age of r is z years.
$\frac{x + y + z}{3} = 20 \\ x + y + z = 60 \\ z = 60 - (x + y) \: \: \: \: \: \: \: \: \: \: place \: value \: from \: eq1\\ z = 60 - 40 \\ z = 20$
after 10 years the age of r will be: 20+10= 30 years