The key to learning the math fact families is lots of short bursts of practice. Here’s another way to vary the practice using these self-checking windows that cover the subtraction fact families through 12. Each math operation packet comes with five 8.5x 11 inch two-sided wipe-on/wipe-off cards that are three-holed punched for easy storage in binders. The windows are larger to make filling in the answers easier. The practice uses mixed review to help ensure mastery.Self-checking answer key.