A project can be completed if A first works in 7 days, then A and B work together in 5 days, and if B works in 7 days first, then A and B work together in 8 days. Now A and B are working on this project at the same time, how many days can it be completed?
一项工程，如果甲先做7天，接着甲、乙合做5天可以完成，如果乙先做7天，接着甲、乙合做8天也可以完成．现在甲、乙同时做这项工程，多少天可以完成．
<gadget id="calculator">10 / 4</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">1 / (5/2)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">12 / (2/5)</gadget>
<output>30</output>

<gadget id="calculator">30 + 5</gadget>
<output>35</output>

<gadget id="calculator">35 * (2/5)</gadget>
<output>14</output>

<gadget id="calculator">1 / 14</gadget>
<output>1/14 = around 0.071429</output>

<gadget id="calculator">1 / 35</gadget>
<output>1/35 = around 0.028571</output>

<gadget id="calculator">(1/14) + (1/35)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 / (1/10)</gadget>
<output>10</output>

<result>10</result>