A person has 5 keys and a door key, but he forgets which one it is, so he tries each one, what is the probability that the third one will open the door.
某人有5把钥匙，一把房门钥匙，但是忘记是哪把，于是逐把试，恰好第三把打开门的概率是多少．
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<result>1/5 = around 0.2</result>