Have you ever seen those little wooden dolls that split open to show another little doll inside, then another inside that one? Ever wonder what they’re called? They’re matryoshka dolls, which were first made in Russia starting in 1890. Matryoshka means “little mom,” and the outside doll is always a woman, but the inside ones can be boys or girls. The big one is usually painted with colorful loopy designs, and if you twist the top half off the bottom, inside is another version of herself, except smaller. Inside that one there’s a third even smaller one, then another inside that one, and so on till you get down to the last teeny doll, which usually doesn’t open at all. Sometimes even the baby has the pattern, because she likes to look good whenever she actually gets out.
Wee ones: If your matryoshka doll has 4 more dolls inside it, how many dolls are in the set all together?
Little kids: If you’re counting your 7 matryoshka dolls and you’ve counted 2 so far, what numbers do you say to count the rest? Bonus: If every other doll in a set of 9 is a girl, how many boys are there? (Remember: the biggest matryoshka is always a girl!)
Big kids: If you have 6 dolls and the biggest doll is 6 inches tall, and each doll is 1/2 inch shorter than the next, how tall is the baby? Bonus: How many ways could 3 doll sets give you 10 dolls, if every set has at least 2? (Don’t worry about the order, just the combinations of sizes.)
The sky’s the limit: If you have a 10-doll set, then get a 9-doll set, an 8-doll set, and so on down to a single wooden baby, how many separate dolls do you have in total? Can you come up with a shortcut to figure it out?
Wee ones: 5 dolls in total.
Little kids: 3, 4, 5, 6, 7. Bonus: 4 boys.
Big kids: 3 1/2 inches tall. You go down 5 steps to get to the baby, or 2 1/2 inches. Bonus: Just 4 ways: 2-2-6, 2-3-5, 2-4-4, and finally 3-3-4. All other combinations are just those in a different order.
The sky’s the limit: 55 dolls. This is a “triangle number,” meaning you’re adding a string of numbers that could stack on each other like a pyramid: 1 on top, then 2 under it, then 3 under those. Notice that when you add 1+2+…+9+10, the 10 and the 1 make 11, and so do the 2 and 9, and the 3 and 8…you end up adding 5 11’s, giving you 55.
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.