2010-II-35

Ans: A
The mean of the distribution

$\begin{array}{cl}
= & \dfrac{10\times 5 + 20 \times 10 + \cdots + 50\times 10}{ 5 + 10 +\cdots +10} \\
= & 32.5
\end{array}$

Hence, the standard deviation of the distribution

$\begin{array}{cl}
= & \sqrt{\dfrac{5(10-32.5)^2 + \cdots + 10(50-32.5)^2}{5 + 10 + \cdots + 10}} \\
= & 11.636~866~7 \\
\approx & 12
\end{array}$