If a rope is wrapped three times around the tree, the difference is 30 centimeters. If it is wrapped four times, the difference is 40 centimeters. How long is the rope in centimeters?
用一根绳子绕树三圈余30厘米，如果绕树四圈则差40厘米，绳子长多少厘米．
<gadget id="calculator">30 + 40</gadget>
<output>70</output>

<gadget id="calculator">4 - 3</gadget>
<output>1</output>

<gadget id="calculator">70 / 1</gadget>
<output>70</output>

<gadget id="calculator">70 * 3</gadget>
<output>210</output>

<gadget id="calculator">210 + 30</gadget>
<output>240</output>

<result>240</result>