Let the no. of people who have listened to Pink Floyd be n(P) = 40
Similarly, the no. of people who have listened to Metallica be n(M) = 30
and the no. of people who have listened to John Denver be = n(J) = 10
Also given that the no. of people who listen to all three i.e. n(P ∩ M ∩ J) = 10
We have to find the no. of people who have listened to only Pink floyd.
Let that no. be x.
then, x = (40-30) + (40-20) - 10
Since, (Demorgan's law - A - (B∩C) = (A - B) ∩ (A - C) )
So, x = 20