Why Did the Cheetah Cross the Bridge?

Here's your nightly math! Just 5 quick minutes of number fun for kids and parents at home. Read a cool fun fact, followed by math riddles at different levels so everyone can jump in. Your kids will love you for it.

Why Did the Cheetah Cross the Bridge?

December 30, 2017

Fast and slow animals can be any size. A cockroach and a snail are both small, but that zippy roach will leave the snail in the dust. And the two-toed sloth, which moves only a few inches a day, can’t keep up even with the tiny snail! So our friend Troy W. wondered, how long would it take a mouse to cross the super-long bridge near his house, and how long would it take a cheetah? Troy lives right near the Thi Nai Bridge, the longest sea bridge in the whole country of Vietnam. It’s 8,127 feet long (2,477 meters), or about 1 1/2 miles. Well, mice can run up to 8 miles an hour, pretty speedy for their size: that’s faster than we humans walk, but slower than we run. 1 1/2 miles is 3/16 of 8 miles, so the mouse could cross it in 3/16 of 1 hour — about 11 minutes. The cheetah, though, could blast across at a steady 60 miles an hour (a mile a minute), so would take only 1 1/2 minutes! That leads to our next question: why did the chicken cross the bridge?…

Wee ones: Who’s faster, you walking at 4 miles an hour, or a mouse moving at 8 miles an hour?

Little kids: If you, the mouse and the cheetah all want to cross, but only 2 of you can go at a time, how many ways can you pair up? (Don’t worry about the order, just who’s with whom.) Bonus: If the cheetah crosses in 2 minutes and the mouse takes 11 minutes, how long does the cheetah have to sit and wait for the mouse?

Big kids: If you run across that 1 1/2-mile bridge and then run back, how far have you run?  Bonus: If you can ride a bike at 30 miles an hour, how long would it take you to ride across and back? (Hints if needed: An hour has 60 minutes…and what fraction of that will you need?)




Wee ones: The mouse is faster!

Little kids: 3 ways: you and mouse, you and cheetah, cheetah and mouse.  Bonus: 9 minutes.

Big kids: 3 miles.  Bonus: 6 minutes. 3 miles is 1/10 of the 30 miles you can do in an hour, so you need 1/10 of an hour.

And thank you Troy for the cool fact about your country’s big bridge!

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About the Author

Laura Overdeck

Laura Overdeck

Laura Bilodeau Overdeck is founder and president of Bedtime Math Foundation. Her goal is to make math as playful for kids as it was for her when she was a child. Her mom had Laura baking before she could walk, and her dad had her using power tools at a very unsafe age, measuring lengths, widths and angles in the process. Armed with this early love of numbers, Laura went on to get a BA in astrophysics from Princeton University, and an MBA from the Wharton School of Business; she continues to star-gaze today. Laura’s other interests include her three lively children, chocolate, extreme vehicles, and Lego Mindstorms.

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