"Mean" stands for "average", so if you add the mass of all the players together, then divide by the amount of players, you get 82kg. [ (number of players) x (mean mass) = (sum of mass) ] which is ... [ 19 x 82 = 1558 ] To find the new mean, rearrange then update this formula. Since you want to find the mean mass, you want a formula that has "mean mass = ..." , so you divide both sides by the number of players to get... [ (mean mass) = (sum of mass) / (number of players) ] Add the new player's mass, 93, to the "sum of mass", add 1 to the "number of players", then plug these new numbers into the formula to get the new mean. [ (mean mass) = (1558 + 91) / (19 + 1) = 82.45kg ]