Practice subtraction math facts daily to work on the skills necessary for memorization with your students. I use this with both my struggling students and with my students who are working on mastery. I give my students who are struggling, manipulatives to work out the problems and help them with memorization of the math fact. I give them about 10-15 minutes daily and they can also practice independently when finished with all other work. The answer key is included at the end of the document (pages 11-22).