[{"weight":70,"height":180},{"weight":80,"height":179},{"weight":100,"height":169}]

db.collection.aggregate([{$project:{cal:{$divide:[{$toDouble:"$weight"},{$pow:[{$divide:[{$toDouble:"$height"},100]},2]}]}}},{$group:{_id:1,avgBmi:{$avg:"$cal"},minBmi:{$min:"$cal"},maxBmi:{$max:"$cal"}}}])