Question

Kyra observed that 85% of the students at a library liked reading mystery stories. If 30 students at the library did not like reading mystery stories, the number of students at the library who liked mystery stories was _____. (only put numeric values, no other symbols)

Answer 1

It is said that 85% of the students liked reading mystery stories. This means the remaining 15% don't like to read these stories. If there are 30 students belonging to the second category, then the number of students who liked reading mystery stories is (30/0.15)*0.85 equal to 170 students out of 200 students. 

Answer 2

The first thing we must do is find the percentage of students who do not like to read mystery stories.
 We have then:
 $100-85 = 15$
 Then, to find the number of students who like to read mystery stories, we can make the following rule of three:
 15% ----------------> 30 students
 85% ----------------> x
 From here, we clear the value of x.
 We have then:
 $x = (85/15) * (30) x = 170$
 Answer:
 the number of students at the library who liked mystery stories was 170