Riddles, brain puzzles and mathematical problems

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So it will (probably) be 2/3 * 2/3 *2/3 + 2/3 * 2/3 - (2/3)^5 =

72/243 + 108/243 - 32/243 = 148/243 =~ 0,609 =~ 60,9%

Just so I can die in peace - what is it you're substracting there? The probability that all switches are closed I presume? In that case I'm relieved because it means I merely misunderstood the drawing instead of having forgotten everything I learned about math.
I thought the cables continued from the corner points to the respective poles, in which case it wouldn't matter if both lines are closed or only one.
(Which IS the only thing that would make sense since there's no power source specified... but anyway.)