Ms. Toehank's class measured the volume of a rock. The starting volume of the water is 30 ml and the ending volume is 40 ml. What is the volume of the object?
The question is probably about the water displacement method for measuring the volume of an irregularly shaped object. The measurement is done by using a graduated cylinder with a set amount (volume) of water in it. In this case the graduated cylinder initially contains 30 ml of water. The rock is then added to the water in the cylinder. As long as the rock is completely submerged (covered by the water) the new measurement of the water level will include the volume of the water plus the volume of the rock (in your problem, 40 ml). The volume of the rock must then be the final volume (volume of rock plus volume of water) minus the initial volume of the water alone:
40 ml - 30 ml = 10 ml
The volume of the rock must be 10 ml which is equal to 10 cubic centimeters `(10 cm^3).`