If 150 empty water bottles weigh 4.5lbs, first let's divide 4.5 by 150 to find pounds per water bottle:
4.5lbs/150 water bottles = 0.03 lbs per water bottle.
Now that we know that each empty water bottle weighs 0.03lbs, we can multiply that number by 90 (the number of water bottles) to find our their weight since the unit "water bottles" will cancel out.
0.03lbs per water bottle x 90 water bottles = 2.7lbs
