### The average age of group of 20 girls is 15 years and that of another group of 25 boys it is 24 years. The average age of the two groups mixed together is :

A. 19.5 years B. 20 years C. 21 years D. 21.5 years Answer: Option B
According to the question, The average age of 20 girls is = 15 years The average age of 25 boys is = 24 years $\begin{array}{l}\begin{array}{l}\text{∴ Mixed average}\\ =\Large\frac{20\times15+25\times24}{45}\\ =\Large\frac{300+600}{45}\end{array}\\ =\Large\frac{900}{45}\\ 20\text{ years }\end{array}$