It takes 7 days for wild ducks to fly from the South China Sea to the North Sea, and it takes 1/3 of the whole journey for wild geese to fly from the North Sea to the South China Sea in 3 days. Now they take off from the South China Sea and the North Sea at the same time, how many days will they meet again?
野鸭从南海飞往北海需要7天，大雁从北海飞往南海3天可飞全程的(1/3)，现在它们分别同时从南海、北海相对起飞，多少天后能够相遇？
<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(1/3) / 3</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">(1/7) + (1/9)</gadget>
<output>16/63 = around 0.253968</output>

<gadget id="calculator">1 / (16/63)</gadget>
<output>63/16 = around 3.9375</output>

<result>63/16 = around 3.9375</result>