if 1/5 of a song is played in 1/15 of a minute, we need to find out how much time is used to play 5/5 = 1 of a song, that is the whole song, we can use a simple rule of three:
1/5 ---> 1/15
1 -----> x
x = (1)(1/15)/(1/5)
x = 1/3
therefore, to play an entire song 1/3 of a minute is needed