Arturo recorded the grade-level and instrument of everyone in the middle school School of Rock below.
Seventh Grade Students
Instrument # of Students
Guitar 4
Bass 5
Drums 3
Keyboard 5
Eighth Grade Students
Instrument # of Students
Guitar 6
Bass 8
Drums 2
Keyboard 15

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