There are 500 telephone sets, sell 20% and increase by 20%, how many telephone sets are there at this time
有500台电话机，卖掉20%，再增加20%，这时电话机有多少台
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 + (1/5)</gadget>
<output>6/5 = around 1.2</output>

<gadget id="calculator">500 * (4/5) * (6/5)</gadget>
<output>480</output>

<result>480</result>