One invention saves 30% on fuel; a second, 45%; and a third, 25%. If you use all three inventions at once can you save 100%? If not, how much?
Puzzle provided by Kordemsky: The Moscow Puzzles (Dover)
Solution: No invention can save 100% on fuel, since energy cannot arise from nothing. The correct calculation is not 30% + 45% + 25% = 100%, but:
100% - (100% - 30%) (100% - 45%) (100% - 25%)
= 100% - (70% ´ 55% ´ 75%) = 71.125%.
This assumes the three inventions are independent in effect.
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