50 and 50% are different. 50 is the actual mark you've got, and 50% is the percentage of the full mark for the paper you took. So for example, in a situation where the full mark for a paper is 90, getting 50% means your actual mark is 45 (90*50%=45). However if you get 50 for this paper, you actually have got 56% (50/90=56%). Hope this helps.