Final answer:
Mr. Williams can grade 20 tests in 4 hours, by dividing the total number of available minutes (240) by the time taken to grade one test (12 minutes).
Step-by-step explanation:
If Mr. Williams takes 1/5 hour to grade each test, we must find out how many tests he can grade in 4 hours. To do this, we can use the concepts of unit rates and proportions. Since 1/5 hour is equivalent to 12 minutes, we are essentially asking how many 12-minute intervals there are in 4 hours.
Since there are 60 minutes in an hour, we calculate the total number of minutes in 4 hours by multiplying 4 hours by 60 minutes, giving us 240 minutes in 4 hours. Next, we divide the total minutes available for grading by the time taken to grade one test:
240 minutes / 12 minutes per test = 20 tests.
Therefore, Mr. Williams can grade 20 tests in 4 hours.