The sporting goods store held a National Day promotion. Originally priced at 90 yuan per football, buy 7 and get 2 free. Teacher Zhang bought 9 footballs, how much is each cheaper?
体育用品商店举行国庆促销活动，原价90元/个的足球，买7个送2个．张老师买了9个足球，每个便宜了多少元？
<gadget id="calculator">90 / 9</gadget>
<output>10</output>

<gadget id="calculator">7 * 10</gadget>
<output>70</output>

<gadget id="calculator">90 - 70</gadget>
<output>20</output>

<result>20</result>