There is a batch of goods in the freight yard. If it is transported by 3 large trucks, it can be transported in 4 days; if it is transported by 4 small trucks, it can be transported in 5 days; if it is transported by 20 tractors, it can be transported in 6 days. There are currently 2 large trucks, 3 small trucks, and 7 tractors. After they have been transported together for 2 days, the rest will be transported by tractors and must be transported within 2 days. How many tractors will be used in these two days?
货场有一批货物，如果用3辆大卡车运，4天可以运完；如果用4辆小卡车运，5天可以运完；如果用20辆拖拉机运，6天可以运完．现有2辆大卡车，3辆小卡车，7辆拖拉机，他们共同运2天后，剩下的改由拖拉机运，而且必须在2天内运完，这两天要用多少辆拖拉机？
<gadget id="calculator">1 / 3 / 4</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 2</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / 5 / 4</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) * 3</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">1 / 6 / 20</gadget>
<output>1/120 = around 0.008333</output>

<gadget id="calculator">(1/120) * 7</gadget>
<output>7/120 = around 0.058333</output>

<gadget id="calculator">(1/6) + (3/20) + (7/120)</gadget>
<output>3/8 = around 0.375</output>

<gadget id="calculator">(3/8) * 2</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 - (3/4)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/120) * 2</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/4) / (1/60)</gadget>
<output>15</output>

<result>15</result>