The Bridge Riddle.
The Bridge Riddle.

Logic puzzles

The good thing about online puzzles is that there are sites that use videos and explain the logic like a good teacher would, writes Shekhar Bhatia

This logic puzzle has been doing the rounds for some time. It’s called “The Bridge Riddle", and this is how it goes: You are a young explorer on a remote island with three other people—a fellow lab assistant, a janitor, and an old and doddering professor. It’s late in the night and you are being chased by a bunch of evil zombies. The only way out of the island is across a rickety rope bridge over a deep gorge.

While you can dash across the bridge in a minute, the lab assistant is a bit slower and will take 2 minutes, the janitor 5 minutes and the professor a whole 10 minutes. The bridge can only hold two people at a time. When two people cross the bridge together, they must move at the slower person’s pace.

It’s dark, and you have just one tiny lantern between the four of you. So a person who has crossed the bridge has to send the lantern back in order to lead another person across.

By the professor’s calculation, the zombies will catch up with you in 17 minutes. You have just that much time to go across and cut the ropes of the bridge. Now figure out a way to get everyone safely over to the other side before the zombies catch you.

Remember: There is no trick; just logic.

When I was young, my father, who loved math and science puzzles, once gave my cousins and me a variation of this puzzle to solve. It was about a man who had to take a fox, a chicken and a bag of grain across a river in a boat. The boat could carry the man and only one of the three items at a time—the fox, the chicken or the bag of grain. He couldn’t leave the fox with the chicken, or the chicken with the grain, and so on.

My father didn’t give us any clues. We had to either put our mind to it and solve the puzzle, or accept defeat and ask someone who had worked out the answer. There were no computers, no Internet, no Google to help us.

Back then, if you were interested in puzzles, you had few options: You asked like-minded friends or bought/borrowed books. I still have the Soviet-era copy of Fun With Maths And Physics by Y.I. Perelman, printed by Mir Publishers, Moscow. My father bought this gem of a book in 1984 for 25 only.

That’s about the only book of puzzles I possess. I still attempt an occasional puzzle, but mostly the ones that have gone viral online—and more out of curiosity than as a challenge.

What I like about online puzzles is that there are sites that use videos and explain the logic like a good teacher would. It’s a marvellous experience. There’s a beautifully animated video version of “The Bridge Riddle" by TedEd, an educational website, on YouTube. I had no idea that online puzzles were so popular till I saw the number of views for this video: over two million at last count.

Take, for instance, this math problem titled “When is Cheryl’s birthday?" that went viral earlier this year. This is how it goes: Albert and Bernard just became friends with Cheryl, and they ask her, “When is your birthday?" Cheryl gives them a list of 10 possible dates: 15 May, 16 May, 19 May, 17 June, 18 June, 14 July, 16 July, 14 August, 15 August and 17 August.

Then she tells Albert the month of her birthday, and Bernard the date of her birthday. And then the two tell each other:

Albert: I don’t know when Cheryl’s birthday is, but I know that Bernard does not know too.

Bernard: At first I didn’t know when Cheryl’s birthday is, but I know now.

Albert: Then I also know when Cheryl’s birthday is.

So when is Cheryl’s birthday?

The problem was set for 14-year-old children in Singapore. I read it again, and realized it’s beyond me. But out of curiosity I looked up the answer and, in the clutter of sites, found two that stand out for their clearly illustrated, step-by-step videos—Numberphile at YouTube and the Khan Academy. A video done by a good teacher makes all the difference.

So if you enjoy puzzles, let me leave you with the one called “The Three Switches":

The Three Switches.
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The Three Switches.

Hint: Think like Sherlock Holmes and not like Albert Einstein.

Shekhar Bhatia is a science buff and a geek at heart.

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