[{"_id":ObjectId("5ef9a4149dd4092813097df6"),"numchoices":[7,7,8],"totchoices":[7,7,8]}]

db.collection.aggregate([{$project:{result:{$map:{input:{$zip:{inputs:["$numchoices","$totchoices"]}},in:{$add:[{$arrayElemAt:["$$this",0]},{$arrayElemAt:["$$this",1]}]}}}}}])