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A bag contains 6 white and 4 black balls. Two balls are drawn at random. The probability that they are of the same colour, is
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$7 / 15$
The number of ways to select 2 balls out of 10 $={ }^{10} C_2$
Number of ways to select 2 balls both white $={ }^6 C_2$.
Number of ways to select 2 balls both black $={ }^4 C_2$
$\therefore$ The required probability $=\frac{{ }^6 C_2+{ }^4 C_2}{{ }^{10} C_2}=\frac{7}{15}$
Number of ways to select 2 balls both white $={ }^6 C_2$.
Number of ways to select 2 balls both black $={ }^4 C_2$
$\therefore$ The required probability $=\frac{{ }^6 C_2+{ }^4 C_2}{{ }^{10} C_2}=\frac{7}{15}$
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