in a group of 35 pupils, 11 play the flute only

in a group of 35 pupils, 11 play the flute only. 8 play the piano only. 15 play neither instrument. a student is selected at random. what is the probability the student plays both instruments?

assuming that a 3rd instrument is not in question, p(a randomly selected student plays both the flute and the piano) = 1/35
 
 
35-15=20, 20-11-8=1
 
a2