The shape that is generated is a Cone.
A triangle, when rotated about one of it's side will generate a solid in a form of a Cone. The cone could be a hollow one or a solid filled one, depending on the properties of the triangle being rotated.
The chance that any given bulb is on is equal to 1/2. The chance that all twenty bulbs are on at the same time is
8 divide by 5
X=4-3 and that's the right answer
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