Given that number of white balls =10 Number of green balls =9 and Number of black balls =7 ∴ Required probability =(10+1)(9+1)(7+1)−1 =11.10.8−1=879 [∵ The total number of ways of selecting one or more items from p identical items of one kind, q identical items of second kind; r identical items of third kind is (p+1)(q+1)(r+1)−1]