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, which is 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.
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.