Okay, let n be the number of songs. k of which are polly. L is the number of shuffles, H is the number of songs going to be played of n. B is the number of H of n are k. So far we have:

k= [n x L / (H - B + k) x k^2] / radical n^2.

Follow? Now if we want the probality of n for L with respect to H as B with a quotient of k, we'll need to define a few more varibles. G is k / B, and Y is the number of songs played minus songs that will play divided by the square of total songs. S is y + (k^2 x G). So,

H= S^2(Y^2 - cos(B-G) + k^2).

Now if we multiply that times the square root of the serial number we find that it is a 1 / 1 chance that k= BS.