Crystal Reports Cross tab averages

Message

   Hi Guys. 

I have a report in Crystal 2008 

Im using a cross tab to display students with there marks. 

on the end i have a SUM of all the marks and a average. 

but where the trick comes in the mark is returned from the stored procedure as a string, because if a student did not write an exam it must display 'xx' instead of a '0' or '45' or whatever the mark may be. 

so i created a formula field with the folowing code: 

/////////////////////////////////////////////////////////// 
Local NumberVar MarkInt; 

If NumericText({sp_Register_Reports;1.Mark}) Then 
ToNumber({sp_Register_Reports;1.Mark}); 

/////////////////////////////////////////////////////////// 

And for the SUM it works great. 

but now on the AVG it doesnt work correctly. 

example there is supposed to print 21 guys on a list. 
and their marks sum up to be 627, but 2 of those students did not write exam it throws the average out. 

now : 627 / 21 = 30 
should be : 627 / 19 = 33 

How can i tell the avg to only COUNT the students that has a mark and not an 'xx' to use for the avg? 

Please Help 