Cut a rope 21.3 meters long into three pieces. It is known that the first section and the second section are 12.85 meters long, and the second section and the third section are 15.7 meters long. How long is the second piece of rope?
把一根长21.3米的绳子截成三段．已知第一段与第二段共长12.85米，第二段与第三段共长15.7米．第二段绳子长多少米？
<gadget id="calculator">15.7 - 21.3</gadget>
<output>-5.6</output>

<gadget id="calculator">12.85 + (-5.6)</gadget>
<output>7.25</output>

<result>7.25</result>