It is necessary to build a canal with a length of 1800 meters. After five days of work, the ratio of the length of the repaired to the unrepaired is 1:3. At this pace, how many days will it take to complete the canal?
要修一条长1800米的水渠，工作五天后，修的长度与未修的比是1：3，照这样的进度修下去，还要多少天才能修完这条水渠？
<gadget id="calculator">1 + 3</gadget>
<output>4</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

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

<gadget id="calculator">1_800 * (1/20)</gadget>
<output>90</output>

<gadget id="calculator">1_800 / 90</gadget>
<output>20</output>

<gadget id="calculator">20 - 5</gadget>
<output>15</output>

<result>15</result>