Text Box and Combo box binding

Question

I need a logic to implement

let consider only two controls textbox and combobox with 4 option.

Option 1 multiply by 1

Option 2 multiply by 2

Option 3 multiply by 3

Option 4 multiply by 4 

for example user enters 1000 in the textbox and select option 2 in combobox.

 

the value 2000 (which is 1000 x 2) should display in the same text box.

 

When the user again changes the option in combobox with option 3.

 

the value to result in the text box as 3000 (i.e, 1000x3) it should not take the current value in the text box for calculation.. it should take the user entered value.