A rope uses half of its total length for the first time, and 2.5 meters for the second time, leaving 8 meters left. How long is this rope?
一根绳子第一次用去全长的一半，第二次用去2.5米，还剩下8米．这根绳子原来长多少米？
<gadget id="calculator">8 + 2.5</gadget>
<output>10.5</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 - (1/2)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">10.5 / (1/2)</gadget>
<output>21</output>

<result>21</result>