### The average age of 20 boys in a class is 12 years. 5 new boys are admitted to the class whose average age is 7 years. The average age of all the boys in the class becomes:

A. 8.2 years B. 9.5 years C. 12.5 years D. 11 years Answer: Option D
$\begin{array}{l}\text{According to the question,}\\ \text{Average}\\ =\Large\frac{20\times12+5\times7}{25}\\ =\Large\frac{240+35}{25}\\ =\Large\frac{275}{25}\\ =11\text{ years }\end{array}$