Adding and subtracting fields in a FORM

Adding and subtracting fields in a FORM


Question,
 
I need to add all the following and display, total hours for the days of week that are updated with the total value to be polulate in a emtpy field some thing like = Total_Hours, what is the best way i should do this?
 
Here is What I have for days of the week, i want to total up the days by hours then if the total equals 4hr i will send a out put of free text:
 
       Sunday_Hours
        (
            displayname  =  "Sunday Hours"
            type  =  number
            width  =  20
            maxchar  =  2
            tooltip  =  "Enter total hours for Sunday"
        )






        Monday_Hours
        (
            displayname  =  "Monday Hours"
            type  =  number
            width  =  20
            maxchar  =  2
            tooltip  =  "Enter total hours for Monday"
        )






        Tuesday_Hours
        (
            displayname  =  "Tuesday Hours"
            type  =  number
            width  =  20
            maxchar  =  2
            tooltip  =  "Enter total hours for Tuesday"
        )






        Thursday_Hours
        (
            displayname  =  "Thursday Hours"
            type  =  number
            width  =  20
            maxchar  =  2
            tooltip  =  "Enter total hours for Thursday"
        )






        Saturday_Hours
        (
            displayname  =  "Saturday Hours"
            type  =  number
            width  =  20
            maxchar  =  2
            tooltip  =  "Enter total hours for Saturday"
        )






       
 
Whats the best way i should do this?