How to do in else statement

Question

                Basic    textbox    textbox
                 Example
                 Basic       2720      2193(i want arrive this(2193) amount when i type 2720)

                 in the first textbox i type the amount 2720
                 in the second textbox i want (2720/31)25 = 2193

 
              Basic    textbox    textbox
                 Example
                 Basic       2720  2266(i want to arrive this(2266) amount when i type 2720)

                 in the first textbox i type the amount 2720
                    in the second textbox i want (2720/30)25 = 2266

that divide may 31 or 30 days

for the above how to do in else statement