It rained 0.45 inches on Friday and 1.02 inches on Sunday. Thus, to find how much it rained total between those two days you simply add them. 0.45+1.02=1.47. That's how much it rained total on Friday and Sunday, but you need to know how much more it rained on Saturday. Thus, you would subtract how much it rained on Saturday form the total rainfall on Friday and Sunday. 2.2-1.47= 0.73. It rained 0.73 more inches on Saturday than on Friday and Sunday combined. :)