[{mcq:[{id:1,answer:"A",time:0},{id:2,answer:"F",time:0},{id:3,answer:"A",time:0},{id:4,answer:"B",time:0}],activity:[{id:1,time:21.2},{id:3,time:10.3}]}]

db.collection.aggregate([{$project:{mcq:{$map:{input:"$mcq",as:"m",in:{$mergeObjects:["$$m",{time:{$reduce:{input:"$activity",initialValue:"$$m.time",in:{$cond:[{$eq:["$$this.id","$$m.id"]},"$$this.time","$$value"]}}}}]}}}}}])