[{"Plane":"5546","Time":"55.0",City:"LA"},{"Plane":"5548","Time":"25.0",City:"CA"},{"Plane":"5546","Time":"6.0",City:"LA"},{"Plane":"5548","Time":"5.0",City:"CA"},{"Plane":"5555","Time":"15.0",City:"XA"},{"Plane":"5555","Time":"8.0",City:"XA"}]

db.collection.aggregate([{$project:{Time:1,Late:{$cond:[{$gt:[{$toDouble:"$Time"},15.0]},"$Time",0]},prob:1,Plane:1,City:1}},{$group:{_id:{Plane:"$Plane",City:"$City"},Sum:{$sum:{"$toDouble":"$Time"}},Late:{$sum:{$toDouble:"$Late"}}}},{$addFields:{prob:{"$divide":["$Late","$Sum"]}}}])