The only way you can place the 5 black balls without have two together would be intersperse them between the white balls.
If you look at the above arrangement there are 8 possible locations for the 5 black ball to go and since they are identical, the order in the which they appear is not important. So the answer is 8C5 = 56
gayathri, I still have difficulty to understand your approach.
There are 7 spaces between the 7 white balls plus the one ahead of the first white one, so there are 8 spaces available for the 5 black balls to fit.
How does this correspond to the logic of 8C5, 8 choose 5?
You have 8 empty positions but you have only 5 black balls to fill the space. According the question only the black balls should not be next to each other there is no such restriction for the white balls. So, now the question is similar to how many sets of 5 can you choose from 8?
You use combinations instead of permutation because order does not matter as all the black balls are identical. If the balls had been numbered then you would have done 8P5.