[{"name":"Alex","value":65},{"name":"petr","value":8},{"name":"hr","value":20}]

db.collection.aggregate([{$facet:{hrVal:[{$match:{name:"hr"}},{$project:{_id:0,value:1}}],allValues:[]}},{$unwind:"$hrVal"},{$unwind:"$allValues"},{$set:{"allValues":{"$cond":[{$eq:["$allValues.name","hr"]},"$allValues",{_id:"$allValues._id",name:{"$concat":["$allValues.name","/hr"]},value:{"$divide":["$allValues.value","$hrVal.value"]}}]}}},{$replaceRoot:{"newRoot":"$allValues"}}])