There are 15 identical balls for sale among which 4 are red, 5 are black and 6 are white. If a person buys at least one ball out of these 15 balls, then the total number of ways in which that person can buy the balls is
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When identical objects of different types are selected, count the possible quantities of each type independently and apply the multiplication principle. If at least one object must be chosen, subtract the case where all quantities are zero.