Your formula for sharing rooms doesn't seem to scale well to many room-sharers. How about: P = (houserent * 0.5)/numpeople + (houserent * 0.5 * a)/numrooms Wh

Your formula for sharing rooms doesn't seem to scale well to many room-sharers. How about: P = (houserent * 0.5)/numpeople + (houserent * 0.5 * a)/numrooms Where a=0.5 if you're sharing a room, else a=1. This basically assumes that half your rent is going towards the common areas & utilities and the other half is going towards the room.