Have you ever seen an old-fashioned clock with letters on it instead of numbers? Those Vs and Xs are Roman numerals from long ago. Romans used the letter I for the number 1; V for 5, because it was shaped like your hand making a V (5 fingers); and X for 10, made by crossing 2 hands (10 fingers total). For numbers 1 less than a major number, they put the I before that letter. So IV = 4, and IX = 9. This became messy as numbers got bigger. Well, in a link shared by our friend Talie B., the Human Calculator tells us how Arabic numerals won everyone over. Each number was drawn to have its own number of angles, to make them easy to remember. As we see here, a 1 had 1 angle, a 2 had 2 angles, and the crazy 9 had 9 angles! It all worked, and now Roman numerals show up only to count hours and Super Bowls.
Wee ones: Which number from 1 to 9 is your favorite — and is it more than, less than, or the same as your age?
Little kids: In the number 4162, which digit is biggest? Bonus: How many angles do they have all together? Try adding them up!
Big kids: If you drew a crazy new number with 17 angles, what pairs of numbers could you add to it to get 23? (Don’t worry about the order — just which go with which.) Bonus: Try to guess the mystery number…If you double it, add 5, and double it again, you get 42. What is the number?
The sky’s the limit: If you’re choosing 2 numbers from 1 to 9, how many ways can you pick 2 odd numbers? (Again, don’t worry about the order — but you can’t pick the same digit twice.)
Wee ones: Different for everyone…pick your favorite number, and see if you have to go up or down from your age to get there.
Little kids: The 6. Bonus: 13, since they add up to 13.
Big kids: 3 choices. The pairs just need to add to 6, so they can be 1 and 5, 2 and 4, or 3 and 3. Bonus: 8. You had 21 before doubling, you had 16 before adding 5…so you had 8 before doubling that first time.
The sky’s the limit: There are just 5 odd digits – 1, 3, 5, 7 and 9 – and as you pair each one with other numbers, you cut down the number of pairs left to do with those. So you have 1-3, 1-5, 1-7 and 1-9 (4 choices), then 3-5, 3-7 and 3-9 (just those 3 choices, because you already did 3-1). That leaves 5-7, 5-9 (2 more), and finally 7-9, giving us 4+3+2+1=10.
Thank you again, Talie — and those of you who want to add and multiply really, really fast to wow your friends, watch the video to see how Scott Flansburg the Human Calculator does it!