Since videos are 3 dollars since 1 times 3 is 3, 3 times 15 would be 45 dollars worth of videos. So you could purchase 45 songs after buying 15 videos.
You can start from any of the options mentioned above however make sure to follow the rule of BODMMAS but you can start from (D) 54÷3 if you just really want a section to start with
