How to create a formula in a cell in excel?

Hello!
I have a Microsoft Excel document used to calculate percentage options:
– For items that cost $20 or less I charge 20% but minimum $3
– For items that cost more than $20 but less than $35 I charge 15% but minimum $5
– For items that cost more than $35 I charge 10%, but minimum $8.
I’m looking for a formula to use for both the cell for the amount and the cell for the result, without using a third cell for the percentage.
Does anyone know how to do this?
