Christian recorded the grade-level and instrument of everyone in the middle school School of Rock below.
Seventh Grade Students
Instrument # of Students
Guitar 6
Bass 4
Drums 7
Keyboard 2
Eighth Grade Students
Instrument # of Students
Guitar 4
Bass 15
Drums 13
Keyboard 6

Based on these results, express the probability that a student chosen at random will play the bass as a fraction in simplest form.