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)

First we need to calculate that numeric value. It cannot be done immediately without calculations.

if 85% of studends liked mystery that means that 100% - 85% = 15% didnt like.

Those 15% are 30 that were given in text. Now we can calculate this using proportion:

15%        30 students
85%         x students

we use cross multiplication:
15*x = 30*85
x = 30*85/15
x = 170

Answer is 170

So the first thing we do is to multiply the top equation by 3. the reason i want to do this is i notice there will be a 6y on both equations and can possibly add/subtract them. so we get 6y = -3x + 27. adding that and the second equation we get 6x = 27-15 which is 12. divide by 6 and we get 2. ∴ x = 2. substituting 2 into the first equation we get 2y = -2 + 9 which is 7. divide by 2 and we get 7/2. ∴ y = 7/2, x = 2.