[{"_id":{"$oid":"5f5f280ffa2236115655cb6a"},"Name":"Rovilio Chipman","Last_season":{"year":"2010-2011","goals":10,"assists":1},"Last_season_2":{"year":"2011-2012","goals":1,"assists":12},"Seasons":[{"year":"2012-2013","goals":11,"assists":4},{"year":"2013-2014","goals":6,"assists":2},{"year":"2014-2015","goals":5,"assists":5}]}]

db.collection.aggregate([{"$project":{"seasons_goals":{"$sum":["$Seasons.goals"]},"nb_seasons":{"$sum":[{"$size":"$Seasons"},2]},"total":{"$sum":["$Last_season.goals","$Last_season_2.goals"]}}},{"$project":{"total":{"$divide":[{"$sum":["$seasons_goals","$total"]},"$nb_seasons"]}}}])