The school bought 12 footballs and 30 skipping ropes, which cost 840 yuan. The footballs cost 50 yuan each. How much does each skipping rope cost?
学校买来12个足球和30根跳绳，共用了840元，足球每个50元，每根跳绳多少钱？
<gadget id="calculator">50 * 12</gadget>
<output>600</output>

<gadget id="calculator">840 - 600</gadget>
<output>240</output>

<gadget id="calculator">240 / 30</gadget>
<output>8</output>

<result>8</result>