Total area of 64 small squares of a chessboard is 400 sq. cm. There is 3 cm wide border around the chess board. What is the length of the side of the chess board ?

Answer: Option D

Area of each square :
$$\begin{array}{rl}& =\left(\frac{400}{64}\right)c{m}^2\\ & =6.25c{m}^2\end{array}$$
Side of each small square :
$$\begin{array}{rl}& =\sqrt{6.25}cm\\ & =2.5cm\end{array}$$
Since there are 8 squares along each side of the chess board, we have :
Side = [(8 × 2.5) + 6] cm
        = 26 cm