This is cause Renata wins a $20 gift card to an online music site. After Renata purchases 16 songs, the gift card has a remaining balance of $0. So you get that equation. (For anyone who needs it now.)
$20 bought 16 songs Therefore each song costs $20/16 = $5/4 = $1.25 So: 2 songs would cost 2 * $1.25 3 songs would cost 3 * $1.25 ... x songs would cost x * $1.25
Since Renata started with $20, then her balance (y) after buying x songs would be:
