Answer: 1/35
Explanation:
From the question, in a group that has 35 pupils, 11 of the pupils play flute only, 8 play the piano only and 15 play neither instrument.
The probability that a student plays both instruments will be:
Since a third instrument is not mentioned, P(a randomly selected student plays the flute and the piano)
35-15=20,
This is the total students minus the students that play neither instruments.
We then subtract those that play flute only and those that plays piano only from 20. This will be:
20-11-8 = 1
Probability (a student plays both instrument) = 1/35