[tex]50 \div 20 \\[/tex]
Since you want to find out how many papers Mr. Elliot grades per minute, and he grades 50 every 20 minutes, divide 20 by 20, and you get 1 minute, and now divide 50 by 20, and you get 2.5 papers per minute. Mr. Elliot grades 2.5 papers per minute.
