A basket contains 5 apples, of which 1 is spoiled and the rest are good. If Henry is to select 2 apples from the basket simultaneously and at random, what is the probability that the 2 apples selected will include the spoiled apple?
~$\frac{1}{5}$~
~$\frac{3}{10}$~
~$\frac{2}{5}$~
~$\frac{1}{2}$~
~$\frac{3}{5}$~
一个坏的都没拿到和拿到一个坏的是互斥事件
从5个苹果里任意拿两个苹果,有10种拿法;
从4个好苹果里任意拿两个苹果,有6种拿法;
则一个坏苹果都没拿到(即两次都拿的好苹果)的概率是6/10,即3/5
其中有一个坏苹果的概率是1-3/5=2/5
5个苹果随机拿两个,有10种拿法;其中一个是坏的,有4种拿法;所以概率是4/10=2/5