Adding values from select dropdown list

I have a dropdown list all with some test values:

<label> Select-test-1
[select* id:select1 class:Assessment multiple “Level 2|100” “Level 3|100” “Level 5|100” “Level 7|100”] </label>

<label> Select-test-2
[select* id:select2 class:Assessment multiple “Level 2|100” “Level 3|100” “Level 5|100” “Level 7|100”] </label>

<label> Select-test-3
[select* id:select3 class:Assessment multiple “Level 2|100” “Level 3|100” “Level 5|100” “Level 7|100”] </label>

Based on the users selection I am wanting to add the values together so they dont go over a certain amount.

This problem is very similar to this (they are not using contact form 7): https://stackoverflow.com/questions/29974959/adding-values-from-select-dropdown-with-javascript

Any help would be appreciated.

Thanks