In November, Mr. Lee's students read 50 books. In December, they read 40 books.

What is the percent increase or decrease in the number of books they read?

20% decrease

Step-by-step explanation:

Start by dividing 40 by 50.

40/50 = 0.8

0.8 is the decimal form of 80%. So now we know that 40 is 80% of 50. This means that the remaining 20% is the difference between the books read in November and December.