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Hard word problem

THe weight of a synthetic ball varies directly with the cube of its radius. A ball with a radius of 2 inches weights 0.80 pounds. Find the weight of a ball of the same material with a 3 inch radius.
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1 Answer

The cube of the first radius is
 
2= 8
 
The cube of the second radius is
 
33 = 27
 
Set up a propotion to solve for the missing weight
 
8/27 = .8/x
 
Cross muliply
 
8x = 21.6
 
x = 2.7