February 4, 2020

Brushing your teeth probably feels more like work than play. But your toothbrush itself is kind of fun, right? When you look at it, it has all those little bristles in cute bundles, or tufts. So our fans Samantha and Gilbert O. asked, how many bristles are in the average toothbrush? We can’t be sure they all have exactly the same number, but orthodontists tell us that brushes have 2,500 bristles are grouped into 40 tufts. Does that work out evenly? Do the math to find out!

*Wee ones:* Look at your toothbrush. Pinch a few bristles between your fingers. How many of them can you count?

*Little kids:* Look at your toothbrush. How many tufts are in the longest row? How many other rows have the same number? *Bonus:* How many tufts are there in total? Add up the tufts in each row!

*Big kids:* How many times do you think you brush your teeth in a year? How would you figure that out? (*Reminder if needed:* A year has 365 days.) *Bonus:* Is 2,500 evenly divisible by 40? How will you figure it out?

Answers:

*Wee ones:* Count as high as you can, starting from 1!

*Little kids:* Different for everyone…might be anywhere from 6 to 10. Many toothbrushes have a longer middle row. *Bonus:* Again, different for everyone…add them up, and count up if it helps.

*Big kids:* Different for everyone…if you brush once a day, it’s 365, but twice a day would be 365 x 2. If you think you brush twice every other day and once the other days, you’re halfway between those numbers. *Bonus:* It is not! 40 is 10 x 4, so you have to be able to divide 2,500 by 10, then by 4 and get a round number. 2,500 divided by 10 is 250, but 4 is 2 x 2, so you have to be able to cut 250 in half twice. 250 divided by 2 is 125, an odd number, so that doesn’t work, and 250 isn’t divisible by 4. Therefore, 2,500 isn’t divisible by 40. Another way: 2,000 is divisible by 40, but 500 isn’t, so the two added together can’t be a multiple of 40.

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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