A gym teacher fills a basket with 70 balls. Some of the balls are rubber, and the rest are foam. He has four classes per day. Each class gets out 10 balls at random. At the end of class, they put
the balls back in the basket for the next class to use. The first class pulls out 2 rubber balls and 8
foam balls. The second class pulls out 4 rubber balls and 6 foam balls. The third and fourth classes
both pull out 3 rubber balls and 7 foam balls. About how many of the balls in the basket are rubber?