This is an interesting question which combines the concepts of Rates (Time & Work) and Mensuration. At first sight, it might appear as though it’s a tad difficult, but, as you work your way through it, you will actually realize that it’s not all that difficult.
We do not have any information about the shape of the room. So is it something we have to worry about? Maybe not.
Whatever the shape of the room may be, the volume of a 3 dimensional object may always be obtained by multiplying the floor area (or ceiling area, since both are same) with the height of the room.
Note: A sphere is an exception to this rule.
Here, since the volume of the room is 800 cubic feet and the height of the room is 8 feet, we can conclude that the area of the floor (or the ceiling) = \(\frac{800}{8}\) = 100 square feet.
To paint 100 square feet, John will take \(\frac{100}{¼}\)= 400 minutes.
But, the question says that he took a total of 28 hours i.e. 1680 minutes to paint the ceiling and 4 walls. This means that he took 1280 minutes to paint the walls.
1280 minutes, when converted to hours, represents 21 hours and 20 minutes (\(\frac{1280}{60}\)). Therefore, the correct answer option is A.
In this question, you do not have to worry about the other two dimensions of the room, as long as you have the height and the volume.
But, at least when you start off, you may go in that direction for a brief period of time. As long as you are able to correct course and understand that you have ALL the data needed to find the answer, you’ll end up solving this question in about 2 minutes.
Hope this helps!
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