The length of a hall is 21 m and the width is 14 m. The sum of the areas of the floor and the flat roof is equal to the sum of the areas of the four walls. Find the height of the hall.

Correct option is A

Given:
Length of the hall = 21 m
Width of the hall = 14 m
Formula used:
Area of rectangle = Length × Breadth
Area of four wall of rectangular hall = 2 × (Length + Breadth) × Height
Solution:
Let the height of the hall be h
Area of floor = 21 × 14 = 294 m²
Sum of floor and ceiling = Area of the four walls of the hall
294 + 294 = 2 × (21 + 14) × H
=> 70H = 588
=> H = 8.4 m