Two engineering teams, A and B, repair a section of road. If team A repairs it in 7 days, then team B can complete it in 3 days; if team A repairs it in 4 days, then team B can complete it in 12 days. Now two engineering teams, A and B, are working together. How many days can it be completed?
A、B两个工程队修一段路，如果A队修7天，然后由B队修3天可以完成；如果A队修4天，然后由B队修12天可以完成．现在由A、B两个工程队合修，多少天可以完成？
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">21 + 3</gadget>
<output>24</output>

<gadget id="calculator">1 / 24</gadget>
<output>1/24 = around 0.041667</output>

<gadget id="calculator">(1/24) * 3</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">(1/24) + (1/8)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / (1/6)</gadget>
<output>6</output>

<result>6</result>