To calculate the cost of paper for the room, we first need to calculate the total area of the walls to be covered with paper.
Total area of the walls = Perimeter of the room x Height of the room
= 2(Length + Breadth) x Height
= 2(21m + 15m) x 10m
= 720 square meters
Now, we need to calculate the total length of paper required to cover this area. Since the width of the paper is 2 meters, we need to divide the total area by 2 to get the length of paper required.
Length of paper required = Total area of walls / Width of paper
= 720 square meters / 2 meters
= 360 meters
Finally, we can calculate the cost of the paper as follows:
Cost of paper = Length of paper required x Cost per meter
= 360 meters x Rs. 3.5 per meter
= Rs. 1260
Therefore, the cost of paper 2 meters wide at Rs. 3 1/2 per meter for papering the walls of a room 21 m long, 15 m broad, and 10 m high will be Rs. 1260.