Calculating Volume by way of displacement

In this example I chose the approach of looking at the volume of the liquid in the tank before the rock was put in , then placing the rock in the tank and observing the new height of the liquid. I use this new height to calculate the new volume inside the tank. The difference in the two volumes is the volume of the rock. This is not the most efficient approach, but as an instructor I believe it is the most effective at illustrating the need for subtraction.