A pair of table tennis balls is 14 yuan, buy 5 sets and get 2 free, buy 5 sets at a time, how much is each pair cheaper?
一副乒乓球14元，买5副送2副，一次买5副，每副便宜多少元？
<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">14 * (5/7)</gadget>
<output>10</output>

<gadget id="calculator">14 - 10</gadget>
<output>4</output>

<result>4</result>