From Wikiversity
A variable is any part of an equasion that replaces a number with something that can represent a number. For example, 5 + E = 14. In this case, E is the variable. You may have seen this before as 5 + __ = 14. To solve a variable including problem with one step, you have to get rid of the first number. So for example, take
x − 36 = 45. To get rid of 36, we need to create − 36. We also need to balance the problem on both sides. Therefore, the problem looks somewhat like this: 
. Now you solve the problem vertically. So
36 + ( − 36) equals 0, so we can get rid of that. Then we take 45 − 36 which equals 9. Therefore, x = 9.
. Now you solve the problem vertically. So
