To dig a tunnel, it takes 15 days for engineering team A to dig alone, and 30 days for engineering team B to dig alone. If the two engineering teams dig together, how many days can they finish digging?
挖一条隧道，甲工程队单独挖需要15天，乙工程队单独挖需要30天，如果两个工程队合挖，多少天能挖完？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/15) + (1/30)</gadget>
<output>1/10 = around 0.1</output>

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

<result>10</result>