SOLUTION: If 1 gallon of paint covers 400 square feet, about how many gallons of paint are needed to paint the outside of a spherical tank of diameter 20 feet?

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If 1 gallon of paint covers 400 square feet, about how many gallons of paint are needed to paint the outside of a spherical tank of diameter 20 feet?
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Surface area of a sphere = 4(pi)r^2
Your Problem:
Surfact area = 4(pi)10^2 = 400pi sq ft.
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400pi/400 = pi gallons of paint or approximately 3.14159 gallons.
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Cheers,
Stan H.