An engineering team paved a section of railway. The original plan was to pave 3 kilometers per day, but the actual pavement was 1 kilometer more than the original plan per day. It took 12 days to complete the railway. How many days were originally planned? (Answer with proportional knowledge)
某工程队铺一段铁路，原计划每天铺3千米，实际每天比原计划多铺1千米，实际铺完这段铁路用了12天，原计划用多少天铺完？(用比例知识解答)
<gadget id="calculator">3 * 12</gadget>
<output>36</output>

<gadget id="calculator">36 + 12</gadget>
<output>48</output>

<gadget id="calculator">48 / 3</gadget>
<output>16</output>

<result>16</result>