Due to the number of brackets, the clearest solution
might be a user-defined function like this.

Function Commission(X As Double) As Double
Select Case X
Case Is <= 20000
Commission = 0.055 * X
Case Is <= 25000
Commission = 0.07 * X
Case Is <= 30000
Commission = 0.075 * X
Case Else
Commission = 0.08 * X
End Function

It assumes one rate on all revenue of a given amount,
NOT e.g. a rate of 5.5% on the 1st $20000 and 7%
on $2500 for revenue of $22500.

HTH,
Merjet