Casho recorded the grade-level and instrument of everyone in the middle school School of Rock below.
Seventh Grade Students
Instrument # of Students
Guitar 5
Bass 10
Drums 13
Keyboard 13
Eighth Grade Students
Instrument # of Students
Guitar 9
Bass 10
Drums 10
Keyboard 4
Based on these results, express the probability that an eighth grader chosen at random will play an instrument other than drums as a fraction in simplest form.
