Solution
For this case the total interval is between 5pm to 10 pm that represent:
5 hr * 60 = 300 minutes
And th eperson is onlu attentive between 6 to 730 pm that represent:
1.5hr * 60 = 90 minutes
then the probability of heard the song is given by:
p = 90/300 = 3/10 = 0.3