How to recalculate the cost of rent?

Question

Mathematics

I have my own game server in cs. There are several paid services, for example, admin privileges with different rights. 50-100-150 rubles / month There are situations when a person takes a service for a month for 50 rubles and a week later makes an upgrade to the service for 100 rubles. Please tell me how, in this case, to make a recalculation so that the days that remain for paying for the service are 50 rubles / month. recalculate to the new cost of 100 rubles. I will not give a person 57 days of rent for 100 rubles / month, if he paid 50 rubles for the first month. I hope I made myself clear)) Thank you all for your help

Answer 1

I'm not a mathematician, well, the first thing that came to mind looks like this:
z = the concept of a month (30 days)
c = current tariff (50r / month)
n = new tariff (100r / month)
d = day of the month (7th day)
formula z - d * c / n
30 - 7 * 50/100 = 11.5 days add in the new tariff
Hope correct

Answer 2

Elementary.
If the tariff is 50 rubles / month, then we write off 50 / 30 rubles. (1.666667 rubles) per day.
As soon as I switched to the tariff of 100 rubles / month, we write off 100 / 30 rubles. (3.333333 rubles) per day.
Accordingly, with a starting balance of 50r. in a week, 38.333333 rubles remain on the account, which will be enough for 11 days at the new tariff.