[{rollno:1,first_round:6,second_round:3,third_round:9,total:18},{rollno:1,first_round:5,second_round:3,third_round:9,total:17},{rollno:1,first_round:4,second_round:3,third_round:9,total:16}]

db.collection.aggregate([{$group:{_id:null,total_average:{$avg:"$total"},first_round:{$avg:"$first_round"},second_round:{$avg:"$second_round"},third_round:{$avg:"$third_round"}}}])