The school bought 50 pens with a unit price of 8 yuan and 15 yuan to reward students, and spent 610 yuan. How many more pens with a unit price of 15 yuan are more than those with a unit price of 8 yuan?
学校买单价是8元的和15元的钢笔共50枝奖励给学生，用去610元．单价15元的钢笔比单价8元的钢笔多多少枝．
<gadget id="calculator">750 - 610</gadget>
<output>140</output>

<gadget id="calculator">15 - 8</gadget>
<output>7</output>

<gadget id="calculator">140 / 7</gadget>
<output>20</output>

<gadget id="calculator">50 - 20 - 20</gadget>
<output>10</output>

<result>10</result>