c# in windows form

good day guyz, i have a question about my system , in my system its about electric billing system in  one month in one room in the dormitory the amount of electric bill is 1000 philippine pesos ( *for sample)  that was consumed by the border so  its by divided by two border (for its sample) but it is depends on the day that the border enter in the room , what should i do so that the amount of electric bill is divided by number of border that depends on its day that they enter into a room?