Discussion

A box has 10 black and 10 white balls. What is the probability of getting two balls of the same color?

Answer: C

There are 20 balls in all.

\[ \text{total ways} = {}^{20}C_{2} = 190 \]

\[ \text{same colour} = {}^{10}C_{2} + {}^{10}C_{2} = 45 + 45 = 90 \]

\[ P = \frac{90}{190} = \frac{9}{19} \]

The complement - one of each colour - has probability \( \tfrac{100}{190} = \tfrac{10}{19} \), and the two add to 1.

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