The += assignment operator adds a given numeric expression and assigns it to the given variable.
variable += num.exp
Adds 1 to the variable, x.
x += 1
The statement below:
total += tax
is equivalent of the statement:
total = total + tax
* arithmetic operator, *= assignment operator, - arithmetic operator, -= assignment operator, /= assignment operator, := assignment operator, Arithmetic expressions, Arithmetic operators, Assignment, Numeric expressions, Precedence, precision statement, Relational operators, sum() function, \= assignment operator