So first you do 3,816 divided by 12 , because you need to find how much for 1 so then you can find out 25% off of 1. To check your work put down 318 times 12 to prove that you're right. Which would be 318 times 25 = 79.5. If you use a calculator it won't have the decimals ( if you just do the numbers ), but the rest is above.
If she used all of her yarn, she'd have 24 pieces. If she wanted 1.25 left, she'd have ten less because 24 • .125 = 8, and 1/4 of eight = 2, and 2 + 8 = 10.