How to get number of items in column in crystal reports

Question

I created a students report card from a database using crystal reports but am stuck somewhere as i cant figue out how to get the number of subjects in a crystal report column

I have so far tried DistinctCount({Table.subject}) but it causes an error "The report application server failed"

Here is how the columns look

Subject	Marks
Mathematic	60
English	70
Science	80
Computer	90

 

 I want to get the average marks which means i have to get the total marks divided by number of subject. I already have the total marks but how do i get crystal reports to count the number of subject using the subjects column.

Thanks