The number of digits in the square root of 625685746009 is = ?

Answer: Option C

The number of digits of the square root of a perfect square number of n digits is
$$\begin{array}{rl}& \text{(i)}\frac{n}{2}\text{, if n is even}\\ & \text{(ii)}\frac{n+1}{2}\text{, if n is odd}\\ & \text{Here, }n=12\\ & \text{So, required number of digits}\\ & =\frac{n}{2}\\ & =\frac{12}{2}\\ & =6\end{array}$$