May 8, 2020

Numbers are nice and neat, so they make really cool patterns. If you know those patterns, you can do some pretty cool math tricks in your head. Some tricks let you add and multiply big numbers in your head. It’s easy to multiply a 2-digit number by 11, and to “square” numbers ending in 5 (to square a number means to multiply it by itself). Let’s learn some of the coolest number tricks out there, so you can wow your family and friends!

*Wee ones:* Can you count to 5? Try it!

*Little kids:* What digits do you use to “spell” the number 11? *Bonus:* What is 11 + 11? And what is that new number plus 11? Try counting up by 11s and see how far you get!

*Big kids: *To multiply a 2-digit number by 11, you add its two digits and stick that sum in between the digits in the tens place. 32 times 11 is 352, because 3+2=5. So what is 43 x 11? *Bonus:* When you multiply by 5, that’s half as much as multiplying by 10. So just cut the number in half, and tack on a zero. Quick, what’s 42 x 5?

*The sky’s the limit:* To take any 2-digit number ending in 5 and multiply it by itself, you take the first digit, multiply it by the digit 1 more than it, and then just tack 25 onto the end. 35 x 35 is 1225, because 3 x 4 (the number 1 more than 3) is 12, and then you tack on 25…1225. So what’s 55×55?

Answers:

*Wee ones:* 1, 2, 3, 4, 5.

*Little kids:* A 1 and another 1. *Bonus: *22, then 33, 44, 55, 66, 77, 88, 99…and then 110!

*Big kids:* 473, since 4+3=7 and that goes in the tens place. *Bonus:* 210. 21 is half of 42, then tack on a zero.

*The sky’s the limit:* 3025. 5 times the number 1 more than 5 (which is 6) gives you 5 x 6 = 30. Then tack on a 25, to get 3025.

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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