Use a rope to measure a water well. If the rope is folded three times and hangs to the water surface, there will be 7 meters above the well. If the rope is folded 5 times and hung to the water surface, there will be 1 meter above the well. How long is the rope?
用绳子测量水井，如果把绳子三折垂到水面，井上余7米，如果把绳子5折垂到水面，井上余1米，绳子有多长？
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">21 - 5</gadget>
<output>16</output>

<gadget id="calculator">5 - 3</gadget>
<output>2</output>

<gadget id="calculator">16 / 2</gadget>
<output>8</output>

<gadget id="calculator">8 + 7</gadget>
<output>15</output>

<gadget id="calculator">15 * 3</gadget>
<output>45</output>

<result>45</result>