can't get checkbox to uncheck when dropdown set 0

I'm trying to add these functionalities to my ordering form

1. when checkbox is checked then Qty dropdown value will change to 1
2. when checkbox is unchecked then Qty dropdown value will change to 0
3. when Qty dropdown value set to anything but 0 checkbox will change to checked
4. when Qty dropdown value set to 0 then checkbox will uncheck

I've got most of it working,
but I cant get #4 to work - checkbox won't uncheck when dropdown set to 0.