  Question

A room is 9 m long, 8 m broad and 6.5 m high. It has one door of dimensions 2 m × 1.5 m and three windows each of dimensions 1.5 m × 1 m. Find the cost of white washing the walls at Rs 3.80 per square metre.

Solution

We have, Length of a room = 9 m Breadth of a room = 8 m Height of a room = 6.5 m Area of 4 walls = 2(l + b)h                          = 2(9 m + 8 m) x 6.5 m = 2 x 17 m x 6.5 m = 221 m2 Length of a door = 2 m Breadth of a door = 1.5 m Area of a door = Length  x Breadth = 2 m x 1.5 m = 3 m2 Length of a window = 1.5 m Breadth of a window = 1 m Since, area of one window = Length  x Breadth = 1.5 m x 1 m = 1.5 m2 Thus, Area of 3 such windows = 3 x 1.5 m2 = 4.5 m2 Area to be white-washed = Area of 4 walls − (Area of one door + Area of 3 windows) Area to be white-washed = [221 − (3 + 4.5 )] m2                                         = (221 − 7.5 ) m2 = 213.5 m2 Cost of white-washing for 1 m2 area = Rs. 3.80 ∴ Cost of white-washing for 213.5 m2 area = Rs. (213.5 x 3.80) = Rs. 811.30MathematicsRD Sharma (2014)Standard VII

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