A bag contains $10$ white balls and $15$ black balls. Two balls are drawn in succession. The probability that one of them is black and the other is white is:
$\frac{2}{3}$
$\frac{4}{5}$
$\frac{1}{2}$
$\frac{1}{3}$
Answer is $C$ probability of first ball white and second one black $=\left(\dfrac{10}{25}\right)\times \left(\dfrac{15}{24}\right)$ probability of first ball black and second one white$=\left(\dfrac{15}{25}\right)\times \left(\dfrac{10}{24}\right)$ probabilty = sum of above two probabilities $=\dfrac{1}{2}.$
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