Installation date : Product Installation date 
From date : Product From date 
To date : Product To date
Days : Difference between From date & To date
No. of student : User will enter the value
Rate/Student/Month : User will enter the value
Amount : (No. of student) * (Rate/Student/Month) * (Days/30) 
Remark : Mumber of students (No. of student) @rate/*month (Rate/Student/Month) of Days: (Days)
                  eg. : Number of students 100 @rate/*month 3 of Days: 288

--> On calculate button amount will be reflected as per provided formula
--> On Apply button popup will close and amount & remark will be updated