I was wondering how i can combine two dropdownlist through a button and populate the third dropdownlist with the combined values.

For example i have 

Dropdownlist A for equipment

And dropdownlist B for Quantity

And dropdownlist C for selectedValues

So when i click on a button when both dropdownlist A & B have a selected value.. It will add combine the values into dropdownlist C, for example..

Baseballbat - 10

Chair - 20

Finally if it's not too hard, can you please guide me with how i can implement the function that when i click onto values on dropdownlist C it will highlight/select the values on dropdownlist A and dropdownlist B.. For example when i click "baseballbat - 10" on dropdownlist C it will highlight "baseball bat" on dropdownlist A and "10" on dropdownlist B. And from there when i click onto 11 on dropdownlist B, a change should be updated onto dropdownlist C from baseballbat-10 to baseballbat-11. 

Appreciate the help. Thanks!