Textboxes formula depends on combobox value

Hello, i want to formula the three textboxes and display the result in textbox4 based on the value of combobox

Combobox Values = Cost Budget, Revenue and Actual Hours

Here's the formula per combobox values:

Cost Budget = Funding * Actual Hours / Revenue (display the result in textbox4)
Revenue = Funding * Actual Hours / Revenue (display the result in textbox4)
Actual Hours = Revenue * Cost Budget / Funding (display the result in textbox4)

Attached is the sample.