A has 300 bags (comprising some 100 lb and some 200 lb) with an average of 180 lb. how many bags should A remove to get 140 lb as average weight of the bags.
Is it a selective removal of 200lb bags? (I mean this is obvious, but just to confirm- If you remove bags randomly, then the average would tend to 150 lbs, and would not approach 140).
If n bags of 200lbs each are removed,
180x300 - 200n / (300-n) = 140
Solving for n, 54000 - 200 n = 42000 - 140n => 12000 = 60n => n = 200
Who says elephants can't dance?