I created this fact series when I was taking a Thinking Math class a few years back. The class emphasized the importance for students to be flexible with their use of operations; teaching addition and subtraction together and multiplication and division together. My focus in creating these 15 practice sheets was to build students understanding of mental math and help them make connections between 7+ 8, 15-7 and 70+80. There are 5 sheets in each of three levels ultimately building up to completely mixed practice. Each practice sheet is organized in neat rows to easily and quickly facilitate whole class correction. There is also a place for the student corrector to record his/her initials or student #. In 10 minutes/ day for a few months, I have seen this series yield excellent results in grades 2-6 .