It takes about 20 minutes to grade a student’s paper. how long, in hours, does it take to grade papers for a class of 25 people?

You have to multiply the 20 min by the number of student in the class which are 25. Then take the minutes and devide them by the number of minutes in an hour.

20 x 25 = 500 minutes
500/60 = 8.3333 (round down) 8 hrs

When rates are expressed as a quantity of 1, such as 2 feet per second or 5 miles per hour, they are called unit rates. if you have a multiple-unit rate such as 120 students for every 3 buses, and want to find the single-unit rate, write a ratio equal to the multiple-unit rate with 1 as the second term.