A pile of coal, originally planned to burn 35 kg per day, can be burned for 45 days. After the stove is improved, it can burn 21 kg per day. How many days can this pile of coal be burned for?
一堆煤，原计划每天烧35千克，可以烧45天，改进炉灶后，每天烧21千克，这堆煤能烧多少天？
<gadget id="calculator">45 / 21</gadget>
<output>15/7 = around 2.142857</output>

<gadget id="calculator">35 * (15/7)</gadget>
<output>75</output>

<result>75</result>