We have to do leak adjustments where I work, and they have a long drawn out way of doing it. I think I can streamline the process by creating a formula for excel so whatever amount my boss puts in it will give her the amounts automatically.

Ex:

Water : $43.19
Sewer: $61.86

Total: 105.05

So when someone has a leak we take off one third of the amount and the customer is responsible for the rest and the amount is based off of the bill.
The water rate is $9.99
Sewer rate is $24.59

So what information do I need to create the formula so that my boss can just enter in the amount of the bill and itll tell her how much she needs to take off AND what would that formula look like, because I'm sitting here doing a lot of math for a simple answer and there are thousands of adjustments we need to get through.
Please Help