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Question

A rectangular plot of land measures 45m × 30m. A boundary wall of height 2.4m is built all around the plot at a distance of 1m from the plot. Find the area of inner surface of the boundary wall


Solution



If you are wondering about how the four wall formula has been used. Consider the outer rectangle and say we are drawing the wall over it.  You would agree the length all around is the perimeter and I hope you know that the perimeter of rectangle contains 4 components, there are 2 lengths and 2 breadths that is why the formula is l+b+l+b or 2(l+b).
If you are going to multiply each part by height. We get  lh + bh + lh+ bh. This is same as h (l+b+l+b) = h(2l+2b) = 2h(l+b). If you know factorization you would be able to understand how h is factored out as a common factor.
I hope the solution is much more clear now.

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