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The answer is the HCF of 8m and 6m 40cm. Because the size of the square tile should be such that it must divide both the length and breadth of the room, then only we can use complete tiles.

HCF of 800cm and 640cm = 160cm
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How can we find number of tiles needed
Answer is 20 tiles but how can we find
=Total area of room / Area of one tile = 800*640/(160*160) = 20