the probability that the person chooses the $1 suitcase is .2, .2 for the $10 one, .2 for the $100 and so on. Then if you add up .2*1 + .2*10 + .2*100 + .2*1000 + .2 10000 that will give you the average value that you can go home with. Subtracting 30% from that should give you the amount that the person takes home.
