In this scenario, the constant rate of change (of w with respect to v ) is 8.35 pounds per gallon. Untried Suppose the volume of water in the tank changes from 18 gallons to 31.2 gallons. v = The corresponding change in the weight of water and tank together is pounds. Suppose the volume of water in the tank changes from 8.78 gallons to 19.39 gallons. v = The corresponding change in the weight of water and tank together is pounds