If it costs 8.7 yuan to buy 3 ballpoint pens and 2 pencils, and 6.8 yuan to buy 2 ballpoint pens and 3 pencils, how much is it to buy 7 pencils?
买3支圆珠笔和2支铅笔要8.7元，买2支圆珠笔和3支铅笔要6.8元，则买7支铅笔多少元．
<gadget id="calculator">8.7 + 6.8</gadget>
<output>15.5</output>

<gadget id="calculator">15.5 / 5</gadget>
<output>3.1</output>

<gadget id="calculator">3.1 * 3</gadget>
<output>9.3</output>

<gadget id="calculator">9.3 - 8.7</gadget>
<output>0.6</output>

<gadget id="calculator">0.6 * 7</gadget>
<output>4.2</output>

<result>4.2</result>