Remember basic arithmetic from school? These work just like those.
Table 15-2. Arithmetic Operators
|-$a||Negation||Opposite of $a.|
|$a + $b||Addition||Sum of $a and $b.|
|$a - $b||Subtraction||Difference of $a and $b.|
|$a * $b||Multiplication||Product of $a and $b.|
|$a / $b||Division||Quotient of $a and $b.|
|$a % $b||Modulus||Remainder of $a divided by $b.|
The division operator ("/") returns a float value anytime, even if the two operands are integers (or strings that get converted to integers).
Note: Remainder $a % $b is negative for negative $a.
See also the manual page on Math functions.