Ding Ding and Dong Dong used the same money to buy the same pens. In the end, Ding Ding got 15 pens and Dong Dong got 10 pens, so Ding Ding returned Dong Dong 7.5 yuan. How much is each pen?
丁丁和东东用同样的钱去买同样的笔，最后丁丁得到了15枝，东东得到10枝，于是丁丁还给东东7.5元，请问每枝笔要多少元．
<gadget id="calculator">7.5 + 7.5</gadget>
<output>15</output>

<gadget id="calculator">15 - 10</gadget>
<output>5</output>

<gadget id="calculator">15 / 5</gadget>
<output>3</output>

<result>3</result>