Change Dropdown Checkbox based on other component value

I'd like to put a check in a checkbox if a dropdown next to it has been used.  Let me illustrate:

I have a form that is sent a long with a number of events to subscribers.

It would have a checkbox next to what is being sent. 

[ ] Enclosed is your {dropdown list with multiple policy names}

[ ] Enclosed is a check for {dropdown for various reasons}

It would be great if I could put a check in the checkbox if the user activates and chooses a value in the associated dropdown list.

So if I use the above example and use the second dropdown because they are sending a refund check it would put the check in the CheckBox10 object once I choose the "policy refund" item from that list (or any item not the default value).

Much appreciate any and all advice.

Thanks for looking,