I need a clearer understanding of how to solve: Tanya went to the grocery store to buy some fruit.  She bought 6 pears, 4 Granny Smith apples, 2 pineapples, and a small basket of plums, which were all placed in a single paper bag.  Unfortunately, the bag that the fruit had been placed in had developed a hole.  Therefore, as Tanya walked home, half of the fruit fell out of the bag.  When she later counted the fruit left in the bag, there were only 9 pieces remaining.  How many plums did she buy?