Previously I used 3 simple formulas (if statements) to calculate a
charge, so if E9 =3D=A33000 then the formula for the charge was

=3DIF(E9<=3D1000;E9*1.1;"=A3100") then
=3DIF(AND(E9>1000;E9 <=3D2000);(E9-1000)*0.9;"=A3200")
=3DIF(E9>2000;(E9-2000)*0.5
Total Cost ... of above 3 items plus E9 (=A33000)

Now people are asking to spend a total of =A33000 including the charge,
so now Total Cost is to be =A33000 and then I have to work back to find
E9.

How is the best way to do this please.

Bob