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Explanation:
SA = surface area of the sphere
SA = 4*pi*r^2
SA = 4*pi*8^2
SA = 804.247719
The surface area is roughly 804.247719 sq ft.
I used the calculator's stored version of pi to get the most accuracy possible.
Since 1 paint can covers 75 sq ft, divide that surface area over 75 to find out how many cans you need.
804.247719/75 = 10.72330292
Round up to the nearest integer to get 11
You'll have leftover paint, but it's better to go over the goal than come up short.