How Do I Calculate Total Commission On a Sliding Percentage Scale?

Trying to create a formula to calculate commission in a month.
If I make a company £18,000 in a month commission would be £2599.65.
Commission banding is
0-5,000 – 10%
5001-15,000 – 15%
15,001 or more – 20%
How would you create a formula to calculate the commission due?
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Alex