Discussion
A bag contains 2 red, 3 green and 2 blue balls. Two balls are drawn at random. What is the probability that none of the balls drawn is blue?
Answer: A
The bag holds \( 2 + 3 + 2 = 7 \) balls, of which 5 are not blue.
\[ P = \frac{{}^{5}C_{2}}{{}^{7}C_{2}} = \frac{10}{21} \]
Counting the non-blue balls directly is quicker than working through the red and green cases separately.
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