At a certain food stand, the price of each apple is ¢ 40 and the price of each orange is ¢ 60.  Mary selects a total of 10 apples and oranges from the food stand, and the average (arithmetic mean) price of the 10 pieces of fruit is ¢ 56.  How many oranges must Mary put back so that the average price of the pieces of fruit that she keeps is ¢ 52?