12% Basic calculation

Question

i posted  this question already but there is some mistake in the mistake so again post this question with correct one.

 Basic textbox1 (1000) 12%on basic textbox2 120

 

in this textbox1 if amt 781 greater means 780 amt want to display in textbox2 else 12% on basic (1000) 120 amt want to display in textbox2.
 
for that how to write code using csharp
 
please help me