A batch of coal was shipped from the cafeteria. It was originally planned to burn 210 kilograms per day, which can be burned for 24 days. After improving the stove, this batch of coal can be burned for 28 days. Question: How many kilograms are saved on average per day compared to the original plan after the improvement of the stove?
食堂运来一批煤，原计划每天烧210千克，可以烧24天．改进炉灶后这批煤可烧28天．问：改进炉灶后平均每天比原计划节约多少千克？
<gadget id="calculator">24 / 28</gadget>
<output>6/7 = around 0.857143</output>

<gadget id="calculator">210 * (6/7)</gadget>
<output>180</output>

<gadget id="calculator">210 - 180</gadget>
<output>30</output>

<result>30</result>