<p>Initially, I thought this was easy, but now I'm having some trouble showing this with equations...</p>
<p>There are four people: Bob, Sally, Joe and Sue. They can choose from six different coloured balls (red, orange, green, yellow, purple and blue), but there are some constraints--Bob cannot choose green or yellow, Sally cannot choose yellow or purple, Joe cannot choose blue, and Sue cannot choose yellow, red or orange. Each can only choose one ball.</p>
<p>How many different combinations exist?</p>
<p>Thanks for the help.</p>