Keep addition drills interesting by varying the exercises. There are four separate sections here for youngsters to solidify their understanding of adding three-digit addends. First, they calculate eight vertically aligned equations, regrouping as necessary. Next, learners utilize mental math by solving two horizontally aligned problems. They examine four completed addition problems with missing digits, filling in the correct numbers in the addends and sums. Finally, there are two word problems which give this skill a more applicable context.