Sewing with Math

I often talk to parents about simple ways to help their kids become better Mathematicians (it is part of my job). Things like cooking, shopping, playing card games or dominos. Well now I am going to start including sewing. I did this project a few weeks ago in 10 minutes, seriously 10 minutes! And all with a round tablecloth and some 3 inch black elastic. I got the idea here. Circle Skirt by Made. It does include some math, cheers to anyone who can tell me WHY this math formula works??

Here is the one of the blog Made. She makes the Circle Skirt out of fabric.

Here is my finished product.

I thought I was creative in using a tablecloth but I googled it and many people had the same idea. Table Cloth Skirt

 I am in search for a less colorful and bright tablecloth now to make another one in something I would actually wear. Although I did wear this to my mom's birthday party... but only my nuclear family was there so I had nothing to be ashamed of. Let me know if you try to make one!