section of road in a village is built. After investigation, it is known that if it is done by team A alone, it will take 10 days to complete it, team B will take 15 days to complete it alone, and team C will take 20 days to complete it alone. Due to the tight construction schedule and wanting to complete the project as soon as possible, the village committee decided to invite two engineering teams to cooperate to complete the project. Which two engineering teams should be selected for the least time? How many days can it be completed?
某村修一段公路，经考察了解，如果由甲队单独做需用10天完成，由乙队单独做需用15天完成，由丙队单独做需用20天完成．因工期很紧，想尽快完工，村委决定请两个工程队合作完成，应当选择哪两个工程队合作用时最少？几天可以完成？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/10) + (1/15)</gadget>
<output>1/6 = around 0.166667</output>

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

<result>6</result>