Without the math object you can do simple arithmetic with JavaScript in the same way as in most old-fashioned programming languages.  Create a new page called arithmetic.html.  Directly in the script element put this:

var firstNumber=45;
var secondNumber=13;
var total=firstNumber+secondNumber;
alert('The total is '+total);

Two numbers are placed into variables and added together (+) before the answer is displayed.  Note also the two uses of the plus sign.  The first is to add the two numbers.  The other concatenates (joins) the string in quotes with the value in the variable.  Try the page but you should be able to guess the result.

Copy all of the code and paste it under itself.  Change the arithmetic operator from + to -.   Change the variable name (twice) to a more suitable one (subtracted?).  Change the alert message to something more suitable as well.  Test it.

Do the same again but with * for multiply and / for divide remembering to change the variable names as well.  You should end up with four alert messages giving four answers.