In this case, you already used 310 minutes out of 500 minutes monthly. So the remaining time for your cell phone should be: 500 minutes -310 minutes= 190 minutes.
You still have 1 week this month, that means the 190 mins time must be divided into 1 week or 7 days. Then the duration you of cell phone use for each day should be: 190 minute/7days= 27.14 minutes/day