What is the average number of girls in school A in all the years taken together ?

Answer: Option C

Average number of girls in School A over the years
$$\begin{array}{rl}& =\frac{\left(3.6+4.2+6.9+9.6+12.9+14.4\right)\times 100}{6}\\ & =\frac{51.6\times 100}{6}\\ & =\frac{5160}{6}\\ & =860\end{array}$$