As you were probably taught when you first learned arithmetic, operators have a precedence: rules dictating in what order calculations are made. These are important to understand because they can dramatically affect the end result.
Table 3.2 lists the order of precedence in C for the arithmetic operators, but it's probably easiest just to follow these three rules:
Multiplication and division take place before addition and subtraction.
Operators with the same precedence level are executed from left to right.
Use parentheses to guarantee your results.
|Rules of Precedence|
|+, - (Sign operators) ...|