Excel formula help

I have what is probably a simple school exam question to solve with Excel.  I have two groups of members of an Arts society.  There are 7 in each group.  All full members pay only 85% of any unit cost, divided by 7 in this case whereas the non-members have to pay the full 100% divided by 7.

There are two distinct unit costs, of £400 and £550, i.e., a total of £950.  The calculation outcome must not be in debit but if any excess occurs it will be shared and given back to all concerned

Thought I was good at Excel but this has stumped me and many more of the group too!

Could do with a solution PDQ please, before the arties  start a revolution.

Thanks, Jon