5. Arrange 3 identical black balls and 3 identical white balls in a row from left to right. If, starting from any position (including this position) and counting to the left, the number of black balls is always not less than the number of white balls, such an arrangement is called a "valid arrangement." The probability of a "valid arrangement" occurring is $\qquad$ .
5. $\frac{1}{4}$ Detailed Explanation: The leftmost and rightmost are black and white balls, respectively, and there are $C_{4}^{2}=6$ ways to arrange the four middle positions, excluding the case where the second and third from the right are all black balls. Therefore, the required probability is $\frac{C_{4}^{2}-1}{C_{6}^{3}}=\frac{1}{4}$.