2016年AMC10竞赛B第12题
有五颗球分别标有 {$$1,2,3,4,5$$}, 从中一次性取出两个球，并且将球上的数字相乘，乘积是偶数的概率是多少？
美国AMC10/12->Knowledge Point->Combination->Probability Calculation->Classical Models of Probabilities 课内体系->知识点->统计与概率->概率
The product will be even if at least one selected number is even, and odd if none are. Using complementary counting, the chance that both numbers are odd is $$\dfrac{\left( \begin{array}{l} {3}  {2} \end{array}\right)}{\left( \begin{array}{l} {5}  {2} \end{array}\right)}=\dfrac{3}{10}$$,  so the answer is $$1-0.3$$ which is $$\left(\text{D}\right) 0.7$$. 