Remember basic arithmetic from school? These work just like those.
Table 10-2. Arithmetic Operators
|$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).
See also the manual page on Math functions.