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Puzzle 17
You have a barrel, filled to the top with water, which weighs 150 pounds.
What can you add to the barrel in order to make it lighter?
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Puzzle 18
What comes next in this sequence:
2 · 3 · 5 · 7 · 11 · 31 · 71 · 91 · ?
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Puzzle 19
Which four colours have been mixed up below?
bylv guee rllo iweo elen t
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Puzzle 20
The river Pregel ran through the town of Konigsberg. In the river were two islands, connected to each other and the rest of the city by seven bridges.
The residents of Konigsberg challenged each other to walk through the city, and cross each of the seven bridges exactly once.
Can you find a path that they might have taken?
This is the 'Seven Bridges of Konigsberg' puzzle, tackled by Leonhard Euler in 1736.