Calculations on the text field value

Summary

Calculations on the text field value

Content

Hi All,

Were are trying to do some calculations on the text fields.

There are three text field lets say A,B,C
for example the value entered in the text field A is "18 months".

Now i want to do a calculation for the first "12 months" from text field "A" with the value in text field "B" and a different calculation for other "6 months" from text field "A" with the value in text field "C".

How can we achieve this ? 

Thank You,
Prudhvi