A rope is 21 meters long, if (1/5) is used, there is still the full length ((())/(())), if 6 meters are used, the full length is used ((()) )/(())), the remaining full length ((())/(())), how many meters are left.
一根绳子长21米，用去(1/5)，还剩下全长的((())/(()))，如果用去了6米，用去了全长的((())/(()))，还剩下全长的((())/(()))，还剩下多少米．
<gadget id="calculator">21 - 6</gadget>
<output>15</output>

<result>15</result>