As quilters, we know there's math. And it's in cooking, and shopping, and lots of other places too.
Soapbox Moment: We need to inspire our children, both genders of them, to embrace math. Figure it out, my mother was always saying to me! Double a cookie recipe, buy materials for a project, you need to do it!!
Last week at the quilt shop, I helped several shoppers figure out how much fabric they required for borders and backings. There is very little that's worse than coming up a few inches short on either of those items and not being able to buy what you're missing. Ask me how I know this!
There are many ways to piece your quilt backs. You can Google it or find it on YouTube, or ask at your own local quilt shop. I've seen little wallet cards that help calculate how much you require for any sized quilt. Of course extra-wide fabrics are nice, but not all shops carry them. (Hint, you still have to do math to figure out the length.)
I like the method where you do a diagonal slice to make your extra-long backing piece into a shorter, wider, more useable size. I saw it demonstrated on Fons & Porter, but the formula and all the math are from either John Flynn or Ginny Beyer, depending upon whose web site you read. There's a worksheet from Ginny Beyer that is free here: http://jinnybeyer.com/wp-content/uploads/2017/01/Diagonal-Backing-Worksheet.pdf
There is a good YouTube video here although it sure isn't the only one available!