So, let's put the test scores in fractions. We need them to have a common denominator, so let's use 10.
5/10=b 2/10=a 3/10= c or below
I added up 5/10 and 2/10 and subtracted that from 10/10 because it represents the rest of the test scores. We need to find how many students are 3/10.
We could multiply 3/10 by 30. Or we could divide 30 by 10, and multiply that by 3. Either way, we get 9.
So, 9 students scored a c or below on the test.