This is where you are going wrong. the P.D.F is just the probability that the event occurs. Your list approach doesn't work because the different items in the list have different probabilities attached to them.As I currently understand it, the P.D.F should be the number of times an outcome occurs, divided by the total number of outcomes.
Lets take f(30) as an example
f(30)=p(10p and 20p) + p(20p and 10p)
=(80/200)*(120/199) + (120/200 * 80/199)
Can you do the rest from here?