[{_id:1,quizzes:[5,6,7]},{_id:2,quizzes:[]},{_id:3,quizzes:[3,8,9]}]

db.collection.aggregate([{$project:{adjustedGrades:{$map:{input:"$quizzes",as:"grade",in:[{$add:["$$grade",2]},{$add:["$$grade",3]}]}}}}])