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. which equation represents the relationship between y, the remaining balance on renata’s gift card, and x, the number of songs purchased? 4x + 5y = –100 5x + 4y = 80 4x + 5y = 16 5x + 4y = 20

Firstly we need to find the gradient of the line, which in this case is the cost of one song. If the gift card has $20 and after 16 songs it has $0 that means that the price of one song would be 20/16 = 5/4 (= $1.25).

We know that the y-intercept is (0, 20), since when 0 songs are purchased the gift card has $20. Thus using the equation form y = mx + c we get:
y = 5/4 + 20

Now if we multiply both sides by 4 we get:
4y + 5x = 80
This is the same as the second answer (5x + 4y = 80), thus B is correct :)