A pile of coal was originally planned to burn 4 tons per day, and it can be burned for 280 days. After the stove is remodeled, 0.5 tons per day can be saved. How many days can this pile of coal be saved after the reconstruction? (Solution with proportional knowledge)
一堆煤原计划每天烧4吨，可以烧280天，经过炉灶改建，每天节约0.5吨，改建后这堆煤可以少多少天？(用比例知识解)
<gadget id="calculator">4 * 280</gadget>
<output>1_120</output>

<gadget id="calculator">4 - 0.5</gadget>
<output>3.5</output>

<gadget id="calculator">1_120 / 3.5</gadget>
<output>320</output>

<result>320</result>