Lydia recorded the grade-level and instrument of everyone in the middle school School of Rock below.

Seventh Grade Students
Instrument # of Students
Guitar 7
Bass 13
Drums 2
Keyboard 7



ighth Grade Students
Instrument # of Students
Guitar 11
Bass 8
Drums 12
Keyboard 11




Based on these results, express the probability that an eighth grader chosen at random will play an instrument other than keyboard as a fraction in simplest form.