Many of my students initially dislike mental math simply because they don't know how to do it. This Power Point walks students through each step of compensation after relating it to compensation in addition. For example, 4 x 99. 4 x 100 = 400. 400 - (4 x 1) = 396. As a tip, I first have students try these problems by writing them down and then switch to doing it mentally when they are ready.