You must subtract 10 from both sides. This is called an Inverse Operation. It is used to isolate a variable on one side. In this case, 10 is being added to n. To get it alone, we subtract 10, but we have to do it from both sides because whatever we do to one side has to be done to the other. So: n + 10 - 10 = 100 - 10, n = 90.
