Mrs White has 3 children whose average age is 12 years, and her neighbour Mrs Green has 2 children whose average age is 7 years. the average age of the 5 children is?
First, I recommend posting problems in the appropriate forums. They will get more attention that way.
For this problem, you may use a weighted average to find the mean of the ages $\displaystyle \bar{A}$ for the five children:
$\displaystyle \bar{A}=\frac{3(12)+2(7)}{3+2}=?$