The first rope and the second rope are 14 meters and 16 meters long respectively, and the third and fourth ropes are both 21 meters long. How many meters is the average length of each rope?
第一根绳和第二根绳分别长14米、16米，第三、四根绳都是21米．平均每根绳长多少米？
<gadget id="calculator">14 + 16 + 21 + 21</gadget>
<output>72</output>

<gadget id="calculator">72 / 4</gadget>
<output>18</output>

<result>18</result>