A young man plans to sell umbrellas at the city's Easter Parade. He knows that he can sell 180 umbrellas at $10 each if it rains hard, he can sell 50 if it rains lightly, and he can sell 10 if it doesn't rain at all. Past records show it rains hard 25% of the time on Easter, rains lightly 20% of the time, and does not rain at all 55% of the time. If he can buy 0, 100, or 200 umbrellas at $4 each and return the unsold ones for $2 each, how many should he buy?