You can not see Individual subject marks by just knowing CGPS, It indicates a result of all Your subjects, To know individual subject scores You need to look at the mark sheet.

If You wanr=t to know the percentage score then there should be some conversion factor which needs to be multiplied to get the percentage score, If this score is for CBSE then the conversion factor for CBSE is 9.5 so percentage score would be 9.5*7.8=74.1