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Logic Riddles

Here are some basic logic riddles and puzzles to test your logical reasoning ability against.

Crossing The Lake Puzzle

There are four people in an adventure race that need to get across a lake. They have a small canoe. The rules say that only the slowest person in the canoe can paddle, only one or two can be in the canoe at a time, and they must all cross in the canoe. From practice, they know that:

Ann can paddle across in 1 minute.

Bill can paddle across in 2 minutes.

Sue can paddle across in 5 minutes.

Mike can paddle across in 10 minutes.

How do they get everyone across the river in the fastest time without breaking the rules?

Barber Riddle

A man was in a small town for the day, and needed a haircut. He noticed that there were only two barbers in town, and decided to apply a bit of logical deduction to choosing the best one. Looking at their shops, he saw that the first one was very neat and the barber was clean shaven with a nice haircut. The other shop was a mess, and the barber there needed a shave and had a bad cut besides. Why did the man choose to go to this barber?

logician with some time to kill in a small town decided to get a haircut. The town had only two barbers, each with his own shop. The logician glanced into one shop and saw that it was extremely untidy. The barber needed a shave, his clothes were unkempt, his hair was badly cut. The other shop was extremely neat. The barber was freshly shaved and spotlessly dressed, his hair neatly trimmed.

Coin Puzzle

There are three clay containers. One of them has only pennies from the year 2005 in it, one has only pennies from the year 1975 in it, and the third has an equal number of each. They are labeled "2005," "1975" and "mixed," however, you know that the labels have been switched, so that each container is marked incorrectly. Can you properly label the containers without looking in them, and only pulling one coin out from one container?

Your job is to relabel the containers correctly. Of course you could just look in the containers to find out which labels match, but can you do it without looking into each container? Reach into any one of the containers and select one marble. Can you now correctly label all three containers? If not, select a second marble from any container. What is the fewest number of marbles you need to inspect in order to correctly label each container?

The Triangular Logic Riddle

What is the area of a triangle that has sides of 14, 11 and 25 inches long?

Solution to the "Crossing The Lake Puzzle"

There are five crossings, counting both directions. Bill paddles across with Ann, and returns alone. Then Mike paddles across with Sue, and Ann paddles back. Bill then paddles back with Ann. The total time is 17 minutes, which is the shortest possible, although Ann could have done the second crossing, and Bill from the fourth, and the result would be the same. The crucial point is that Sue and Mike had to make the third crossing.

Solution to the "Barber Riddle"

Since even barbers rarely try to cut their own hair, and there are only two barbers in town, they must cut each other's hair. The one with the neat hair must have it cut by the one with the bad haircut, who must then be the better one, considering his own haircut.

Solution to the "Coin Riddle"

Just pull one coin from the container labeled "mixed." You know that it isn't the mixed container, so if it is a 2005 penny, the container labeled "1975" must be the mixed container, and the one labeled "mixed" is the 1975 container. If it is a 1975 penny, well... you get the point. The logic is the same.

Solution to the "The Triangular Logic Riddle"

The answer is zero, because you can't have a triangle with sides measuring 14, 11 and 25 inches.

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