Explanation:
before buying any songs (0 number of songs) she has $50.
after buying 5 songs she has $37.50 left.
after buying 10 songs she has $25 left.
so, we see, each song costs $2.50 (5×2.5 = 12.5, 50-12.5 = 37.5, 10×2.5 = 25, 50 - 25 = 25).
that ratio continues also for 15 and 20 songs.
so, yes, the problem definition is right, it is a linear function.
the function is
m(x) = y = 50 - 2.5x
"m" standing for "money left".
x = number of songs bought.
the y-intercept is the y-value, when x = 0.
that is in our case 50. or $50.
that stands in this case (as the first sentence in this explanation shows) for the initial amount of money she had before buying any songs.