Calculated field with php

Hi everybody...
I need somethin..

I have a form with some calculated field and I don´t know how to put the calculated data in this field. I want this fields to be displayonly that means that only the data can be put in it by formula not by the user typing..
Ex. I have a grade register form.. and I have 90 in the first grade and 85 in the second I want to calculate the avg. and put this avg in another field. How I do this...

Nestor Valdez...
Answer me in my email
nestorvaldez@hotmail.com

Ill take it from where you have the to grades
firstly you need to do this

$var = ($grade1 + $grade2) / 2

then insert $var into the database.

Ill take it from where you have the to grades
firstly you need to do this

$var = ($grade1 + $grade2) / 2

then insert $var into the database.
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I know that but my problem is :
How do i show this avg var in the browser in the field avg, before I save it in the database.
i want to show the avg var in this field when i left the GRADE2 field. it is possible?