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db.collection.aggregate([{$project:{time:{$split:["$durationHrs",":"]}}},{"$project":{totalTime:{$sum:[{"$multiply":[{"$toInt":{"$arrayElemAt":["$time",0]}},60]},{"$toInt":{"$arrayElemAt":["$time",1]}}]}}},{$group:{_id:null,durationHrs:{$sum:"$totalTime"}}},{$project:{durationHrs:{"$concat":[{"$toString":{$toInt:{"$divide":["$durationHrs",60]}}},":",{"$toString":{$mod:["$durationHrs",60]}}]}}}])