If you buy the same exercise book with the same amount of money, you can buy 4 in the past, but now you can buy 5. How much has the unit price of this exercise book been reduced?
用同样多的钱买同一种练习本，原来可以买4本，现在可以买5本，这种练习本的单价降低了多少
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) - (1/5)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) / (1/4)</gadget>
<output>1/5 = around 0.2</output>

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