At the beginning of a science experiment, Betty had 40 milliliters of solution. During the experiment, the change in the volume of the solution was
–11 milliliters.
What is the final volume of the solution?

Question

At the beginning of a science experiment, Betty had 40 milliliters of solution. During the experiment, the change in the volume of the solution was 
–11 milliliters.
What is the final volume of the solution?

AZ · Answer

So in the beginning of the experiment, she had 40 millimeters

In the experiment, it changes by -11 millimeters

That means she used 11 millimeters of solution during the experiment. You have to subtract 11 from the initial 40 mm

So what is 40 - 11 = ??