In November, the school store sells 120 pens. In December, the store sells only 30 pens. What is the percent of change in pens sold from November to December?

Notice that it goes from 120 to 30 ... so we are dealing with a decrease

percent decrease = (original number - new number) / original number ... * 100

= (120 - 30) / 120 ... * 100

= 90/120 ... * 100

= 0.75 * 100

= 75% decrease <===