Wrap a rope three times around the tree and there is still a gap of 30 centimeters, and four times around the tree is still 40 centimeters away. What is the circumference of the tree in centimeters?
用一根绳子绕树三圈余30厘米，绕树四圈还差40厘米．问树的周长是多少厘米？
<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>

<result>70</result>