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

Answer:

11 years old

Step-by-step 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

$\dfrac{24+x}{5}=7.$

Multiply this equation by 5:

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

The sister is 11 years old.