With the same amount of money to buy the same pen, you can buy 4 pens in the past, but now you can buy 5 pens. How much has the unit price of this pen been reduced?
用同样多的钱买同样的笔，过去能买4支，现在却能买5支，这种笔的单价降低了多少．
<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>