Forms :: Numbers Separated By Comma In Text Box

I have a single 'text' box on a form that captures test scores. The person inputting the test scores puts in all of the scores separated by commas (77, 85, 100, 75.5).. Up to 6 test scores can be placed in this single text box. The field is formatted for text. What I propose to do is take the numbers in the text box and average them out and then display the test scores average in a number field named 'avescores' next to this field in the form. Once the inputer puts the series of scores into the text box, they would hit an 'update' button next to this box to 'update' the 'avescores' box. Right now they use a number pad and add up the numbers manually then place the average into the avescores box....