each song (s) download costs $ 1
each video (v) cost 6 times as much as a song.....so each video costs (6 * 1) = $ 6
u have a credit of $ 60 and u have already purchased 7 videos...
s + 6v = 60.....and v = 7 because he purchased 7 videos
s + 6(7) = 60
s + 42 = 60
s = 60 - 42
s = 18 <== he purchased 18 songs