Reword "Find per-person share of bill among a group" exercise

[k4y3ff]

Currently, this exercise is worded in functions.md and module3.html as follows:

Modify our total-with-tip function, and call the new function share-per-person, that additionally takes in as an argument the number of people in the group for a bill. Have the function return the average bill amount per person.

I remember this being misunderstood by some of the students in the last SF workshop, and I think it's pretty difficult to parse in one go.

Anyone have suggestions for clarifying this exercise?