Try looking at this, it might help you. http://virtualnerd.com/algebra-1/relations-functions/relations/definitions/solution-set-definition (not made by me)
The IQR or interquartile range is the difference between Q1 and Q3, also know as the upper and lower quartiles.
Total: 318
Student tickets: x
Adult tickets: 3x+6
x+(3x+6)=318 -> 4x+6=318 -> 4x=312 -> x=78
3(78)+6 = 234+6 = 240
240 adult tickets were sold