One way to add and subtract mentally is to understand the role place value plays. Too often, students are taught only one way to add and subtract by using regrouping as the algorithm. While this knowledge is important, having the number sense to add and subtract two and three digit numbers (or more!) helps the student to visualize what he or she is doing when problem solving mathematically.
This set of worksheets gives a brief explanation and example of the thought process behind adding and subtracting by multiples of 10. It provides an addition and subtraction practice sheet, as well and a more challenging sheet which includes digits in the one's as well as the ten's places.