To build a 500-meter-long canal, 20% of the total length was built in the first three days. According to this calculation, how many days will it take to complete the canal? (Can you list several formulas)
修一条500米长的水渠，前三天修了全长的20%，照这样计算，修完这条水渠需要多少天？(你能列出几种算式)
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/5) / 3</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">500 * (1/15)</gadget>
<output>100/3 = around 33.333333</output>

<gadget id="calculator">500 / (100/3)</gadget>
<output>15</output>

<result>15</result>