Question
A well is dug 20 ft deep and the mud which came out is used to build a wall of width 1 ft around the well on the ground. If the height of the wall around the well is 5 ft, then what is the radius of the well?
Answer: Option D
:
D
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D
Solution: - Let radius = r feet.
Volume of mud from well: πr2(20) cubic feet.
Volume of wall around well: 5π((r+1)2-r2).
πr2(20) = 5π((r+1)2-r2) => 4r2 - 2r - 1=0 ⇒ r = (√5+1)4
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