The length of a rectangular wooden board is thrice its width. If the width of the board is 120 cm, find the cost of framing it at the rate of Rs 5 for 20cm.

Question

$\begin{array}{11} (a)\; \text{Rs 340} \\ (b)\; \text{Rs 150} \\ (c)\; \text{Rs 240} \\ (d)\; \text{Rs 120} \end{array}$