## Question

Seven white bails and three black balls are randomly placed in a row. The probability that no two black balls are placed adjacently equals

### Solution

7/15

The numbers of ways of placing 3 black balls at 10 place is .

The number of ways in which two black balls are not together is equal to the number of ways of choosing 3 places marked with X out of eight places

*X W X W X W X W X W X W X W X*

This can be does in ways. Thus, probability of the required event is

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