The school brought a pile of coal, burned it in the first week (1/5), and burned 300 kilograms in the second week. At this time, the ratio of the mass of the burned coal to the remaining mass is 3:7. How many kilograms of this pile of coal are left ?
学校运来一堆煤，第一周烧去了(1/5)，第二周烧去300千克，这时烧去的和剩下的质量比是3：7，这堆煤还剩多少千克？
<gadget id="calculator">3 + 7</gadget>
<output>10</output>

<gadget id="calculator">3 / 10</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(3/10) - (1/5)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">300 / (1/10)</gadget>
<output>3_000</output>

<gadget id="calculator">7 / 10</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">3_000 * (7/10)</gadget>
<output>2_100</output>

<result>2_100</result>