Run javascript on multiple selected checkboxes

Hello I am currently writing a web application that calculates a number based on what a user has checked.

You can see it here.

If you go to the link you can see that a person will check a checkbox first and enter a value from the dropdown and type 2 values to get an output.

What I need help with is being able to calculate the value for more than one checkbox at a time.

Right now I can only calculate the value for a checkbox one at a time even if multiple are selected. So basically I need help try to figure how to calculate for more than one checkbox at a time.

I was using an if statement inside my javascript file but thats not giving me the result that I want.