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Question
A classroom is 7 m long, 6 m broad and 3.5 m high. Doors and windows occupy an area of 17 m2. What is the cost of white-washing the walls at the rate of Rs 1.50 per m2.
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Solution
\[\text { Length of the classroom = 7m } \]
\[\text { Breadth of the classroom = 6 }m\]
\[\text { Height of the classroom = 3 . 5 m }\]
\[\text { Total surface area of the classroom to be whitewashed = areas of the 4 walls }\]
\[ = 2 \times \text { (breadth } \times\text { height + length } \times \text { height })\]
\[ = 2 \times (6 \times 3 . 5 + 7 \times 3 . 5)\]
\[ = 2 \times (21 + 24 . 5)\]
\[ = 91 m^2 \]
\[\text { Also, the doors and windows occupy 17 } m^2 . \]
\[\text { So, the remaining area to be whitewashed }= 91 - 17 = 74 m^2 \]
\[\text { Given that the cost of whitewashing 1 } m^2\text { of wall = Rs }1 . 50\]
\[ \therefore \text { Total cost of whitewashing 74 } m^2 \text { of area } = 74 \times 1 . 50 = Rs 111\]
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