How do I calculate bills

[Guest]

Hi,

I live with 5 more roommates and we divide the bills equally. Sometime one of the roommate flies out of the country and then we don't count his days for the calculation (since he doesn't use water and electricity when he's not here).
The thing is, I don't know how to calculate exactly the ratio of the numbers when he is and isn't.
For ex.
If we got 250$ water bill for 60 days for lets say using 50,000 liters, and let's say 5 were for 30 days and 6 for the other 30 days, how do I do the right calculation? I know that the 30 days that we were only 5 we used less water than when we were 6 but how can I know the right numbers?

Thanks!!!

[shyamjayakannan]

Let us assume that each person uses [tex]x[/tex] liters of water everyday. So, the one person who travels used only [tex]30x[/tex] liters of water while the rest used [tex]60x[/tex] liters during the 60 day period.

Cost for each person = [tex]\frac{\text{water used by them}}{\text{total water used}}\times\text{total cost}[/tex]

So, for that one person, cost = [tex]\frac{30x}{30x+5\times60x}\times250=\frac{250}{11}=22.72[/tex]$

And for the rest, cost = [tex]\frac{60x}{30x+5\times60x}\times250=\frac{500}{11}=45.45[/tex]$