Question

Four brothers and their sister have the average age 7. Four brothers have the average age 6. How old is the sister?

Answer 1

11 years old

Explanation:

If four brothers have the average age 6, then the sum of their ages is

`4\cdot 6=24`

years.

Let x years be the age of sister. If four brothers and their sister have the average age 7, then

`(24+x)/(5)=7.`

Multiply this equation by 5:

`24+x=35\Rightarrow x=35-24=11.`

The sister is 11 years old.