I'm not positive what your asking. If P is supposed to stand for probability, then the chance of the non-rap songs being played is calculated by using ratios. There are a total of 22 songs, 13 off which aren't rap. So the ratio is 13/22. Since 13 is a prime number, the ratio can't be simplified. The percent chance that a classical or rap song would play is 59%, which is calculated by dividing 13 by 22 and bringing the decimal 2 spaces to the right.
I hope this helps.