Each of the 30 boxes in a certain shipment with either 10 pounds or 20 pounds, and the average (arithmetic mean) weight of the boxes in the shipment is 18 pounds. If the average weight of the boxes in the shipment is to be reduced to 14 pounds by removing some of the 20-pound boxes, how many 20-pound boxes must be removed?
let x be the number of 10 pound boxes.
therefore [10x + 20(30-x)]/30 = 18
solving this you get x = 6
now setup the equation if y boxes were removed from the 24 (30-x = 30-6 = 24) twenty pound boxes
so you get [60 + 20(24 - y)] /(30- y) = 14
solving for this you get y = 20 boxes for removal.