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, "Sorry-please 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 groceries in the amount specified on their
card. The drawings continue until the store has given away more than \(\$ 500\)
of free groceries. Estimate the average number of winners each week.