UPSC Civil Services 2016 Prelims General Studies Paper II

Let x1,x2,x3,x4 and x5 be the present ages of the family comprising of 5 members 3 years ago,
(x1+x2+x3+x4+x5)−3×5=80
⇒ (x1+x2+x3+x4+x5)−15=80....(i)
Now, let the age of new born baby be x6
Given, Average age of family today = Average age of family 3 years ago
⇒

(x1+x2+x3+x4+x5)−15

5

=

x1+x2+x3+x4+x5+x6

6

⇒

80

5

=

15+80+x6

6

[from (i)]
⇒ x6=1
∴ Baby's age =1 year