In this question, you're trying to find the unit rate in which the teacher can grade tests.
A unit rate is another word for "rate of change".
This means that we have to find how many tests the teacher could grade in 1 minute.
To find the unit rate, divide 18 by 15
18 ÷ 15 = 1.2
This means that Mr. Rivera can grade 1.2 tests in 1 minute.
Answer:
1.2 tests
