6. An urn contains five red chips numbered 1 to 5 , five blue chips numbered 1 to 5 , and five white chips numbered 1 to 5 . Two chips are drawn from this urn without replacement. What is the probability that they have either the same color or the same number?
Answer: $\frac{3}{7}$
Solution: The required probability is
$$
\frac{3 \cdot\binom{5}{2}+5 \cdot\binom{3}{2}}{\binom{15}{2}}=\frac{30+15}{105}=\frac{3}{7} .
$$