A pair of table tennis rackets is 28 yuan, buy 5 pairs and get 2 pairs free, buy 5 pairs at a time, how much is each pair cheaper?
一副乒乓球拍28元，买5副送2副，一次买5副，每副便宜多少钱？
<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">28 * (5/7)</gadget>
<output>20</output>

<gadget id="calculator">28 - 20</gadget>
<output>8</output>

<result>8</result>