How to remove some digits (/20) from column of results to get averages for school reports

Hi, I am a teacher and am hoping that there is a faster way to get the averages of my students' results for school reports. I copy their results from our electronic results system and paste them into excel. They come out in the form 12/20 or 13/20 as all results for this class are out of 20. I know I can get an average by highlighting the row and clicking on a cell to put the average of the rows (I hope to master that bit myself) but the /20 format that the results come out in means that I can't do that to get an average. I have about 10 downward rows of results to average for about 70 students. It normally takes many many hours and I'm wondering if anyone could advise me of a way to set the column to remove the /20 in the rows so that I can get a quick average to speed up the process. Any advice would be much appreciated.