A student has a container with a volume of 1.5 liters. She estimates the volume to be 1.2 liters. By what percent is the student's estimate off?

The answer is 20%. To get this you take the estimated volume, 1.2, and divide it by the total volume, 1.5, to get 0.8. Then you take 0.8 and multiply it by 100 to make it the percent she estimated, 80%. After that you take 100% minus the estimated percent, 80%, to get the percent missing, 20%.