The width of the room is 15 and a half feet. The problem can be solved using the volume formula V = L x W x H. Volume, length and height are given with the values 6,510 cubic feet, 40 feet and 10.50 feet respectively. Use direct substitution to find the width. 6,510 cubic feet = 40 feet x W x 10,50 feet. Combine like terms to isolate the unknown. W = 6510 cubic feet / 420 square feet.