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Can You Solve This River Crossing Puzzle?

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River crossing puzzles are a classic form of logic puzzle. In them, you're provided with a scenario—some number of entities trying to cross a river using a raft or boat—and a set of constraints (typically, some of the entities might eat each other under certain circumstances).

In the TED-Ed video below, we tackle a variant of the puzzle in which a group totaling six, three lions and three wildebeest, need to cross a river using a raft. Only two animals can go at once. The problem is, if the lions ever outnumber the wildebeest, they'll eat them. How can they all cross the river?

The larger question of this puzzle is how should we solve such puzzles? In the video, the narrator walks through this solution, but explains how it can be generalized by drawing up decision trees. At each step of the puzzle, you lay out all the possible options, then cross out any that don't work. As you proceed, the set of possibilities dwindle until you're left with only a few viable paths.

Here are the conditions for this puzzle (also listed in the video):

1. The raft needs at least one animal to paddle it across the river, and it can hold at most two animals.

2. If the lions ever outnumber the wildebeest on either side of the river (including the animals in the boat if it's on that side), they'll eat the wildebeest.

3. The animals can't just swim across, and there are no tricks; the animals have to use the boat as described.

Tune in to see how it's done:

For more on this puzzle, check out this TED-Ed page which explains its relationship to the The Missionaries and Cannibals Problem.

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This Might be the World's Hardest Jigsaw Puzzle
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For advanced jigsaw puzzle players, the standard 1000-piece board is child's play. But imagine if none of those pieces had a cute cat or weeping willow on the front to guide you.

The “Black Hell” puzzle—or “White Hell,” depending on your color preference—by Japanese manufacturer Beverly features a single solid color on its front. Adding an extra layer of frustration, the company claims its micro-sized puzzle pieces are the “world’s smallest,” suitable only for the nimblest of fingers.

One reviewer said it took him 17 months to complete half the puzzle. Another commented, “This puzzle is the devil reincarnated into an inanimate object!”

Although the front of the puzzle is blank, different patterns are imprinted on its back to help users connect the dots. This apparently disappointed one especially masochistic puzzle enthusiast, who wrote that it was “less impossible than I had hoped.”

The recommended age for this puzzle is seven years and up, but it’s safe to say that this devilishly hard task is best left to professionals.

While the Black Hell is certainly in the running for the most challenging jigsaw puzzle, it doesn’t top the chart for ones with the most pieces. Amazon sells a few 9000-piece puzzles, and ones with 5000 pieces are easy to find.

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The Math Puzzle That’s Driving the Internet Bananas
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Here at Mental Floss, we love a good brain teaser—and clearly we’re not alone. From cows and chickens to river crossings, we’ve never met a riddle we didn’t want to solve—even if it was originally meant for a 5-year-old. The Bananas, Clock, Hexagon Viral Logic Puzzle, which math puzzle enthusiast Presh Talwalkar posted to his Mind Your Decisions blog, is the latest riddle to have us admittedly stumped.

According to Talwalkar, 99 percent of the people who attempt to solve the problem fail, leaving the remaining one percent to be dubbed geniuses for figuring it out. Which side will you land on?

The key to solving this puzzle is to look closely. We’ll give you a minute to do just that (or you can start the video below—it will give you a little time before giving anything away).

Now it’s time to come up with your answer. We’ll give you another minute …

 
 
 
 
 
 
 

So what did you come up with?

If you answered 38, congratulations—you might be a genius.

If you answered something else, don’t worry, you’re not alone.

Where most people seem to be going wrong with the problem is by not looking at the images closely enough when attempting to assign a numerical value. Specifically: In the last line of the problem, all of the images—the clock, the bananas, and the hexagon—are all slightly different than the images shown in the previous lines. If you noticed this, then you probably realized that the bunch of bananas in lines one, two, and three have a different value than the fruit seen on line four. Same goes for the clock and the hexagons. Which makes this as much a visual puzzle as it a math problem.

Finish watching the video above for Talwalkar’s detailed explanation of how to solve the problem. Then stump your friends!

[h/t: Mind Your Decisions]

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