PHP/MYSQL Averaging each row

PHP/MYSQL Averaging each row

I have a form with X amount of columns that are labeled C1, C2, C3..to CX.
I also have a column labeled "Average" and another labeled "Target". The
"Average" column finds the average of each row in columns C1 - CX. Is it
possible for the "Average" column to dynamically change while user is
filling in the form? If so, how?
If that is not possible, then how do I have the "Average" column update
its value upon submitting the form with the edited C1-CX values?
Just looking for tips on how to deal with the Averages. I already have
working C1-CX columns with pre-defined fields that save to a MYSQL
database.