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:
15x = 3085
x = 30*85/15
x = 170
Answer is 170