其它
There are three boxes labeled $$A$$, $$B$$, and $$C$$. Box $$A$$ contains $$1$$ red ball and $$4$$ white balls. Box $$B$$ contains $$2$$ red balls and $$3$$ white balls. Box $$C$$ contains $$3$$ red balls. All the balls are the same except for the color. Oscar randomly selects one box and then randomly selects one ball from the selected box. What is the probability that he selects a red ball? 
Overseas Competition->Knowledge Point->Counting Modules->Statistics and Probability
$$\frac{1}{3}\times \frac{1}{5}+\frac{1}{3}\times \frac{2}{5}+\frac{1}{3}=\frac{8}{15}$$ 