[/** 1*/{"_id":ObjectId("57e0ed40828913a99c2ceb46"),"time":20,"amount":40},/** 2*/{"_id":ObjectId("57e0ed40828913a99c2ceb47"),"time":40,"amount":20},/** 3*/{"_id":ObjectId("57e0ed40828913a99c2ceb48"),"time":50,"amount":10},/** 4*/{"_id":ObjectId("57e0ed40828913a99c2ceb49"),"time":10,"amount":5}]

db.collection.aggregate([{$group:{_id:null,amount:{$sum:{"$cond":{"if":{"$and":[{"$gte":["$amount",20]},{"$lte":["$amount",40]},]},"then":"$amount","else":0,}}}}},{$project:{_id:0,amount:1}}])