Can anyone help with this problem? I must be missing a step or something.

29. Free groceries. To attract shoppers, a supermarket runs a weekly contest that involves \"scratch-off\" cards. With each purchase, customers get a card with a black spot obscuring a message. When the spot is scratched away, most of the cards simply say, \"Sorryplease try again." But during the week, 100 customers will get cards that make them eligible for a drawing for free groceries. Ten of the cards say they may be worth $200, 10 others say $100, 20 may be worth $50, and the rest could be worth $20. To register those cards, customers write their names on them and put them in a barrel at the front of the store. At the end of the week the store manager draws cards at random, awarding the lucky customers free gro- ceries in the amount specified on their card. The draw- ings continue until the store has given away more than $500 of free groceries. Estimate the average number of winners each week