Interesting problem Five bags each contain 10 balls. The first bag has yellow balls, the second blue balls, the third red balls, the fourth green balls and the fifth orange balls. All the balls in four of these bags (we don't know which ones) weigh 20 units each and the the balls in the remaining bag weigh 18 units. If we are to determine the bag containing the lighter balls by method of weighing on a single plate scale, what is the minimum number of weighings needed? Describe the procedure.