After a pile of coal is used up (4/7), there are still 150 tons left. How many tons of this pile of coal are there?
一堆煤用去(4/7)后还剩150吨，这堆煤原来有多少吨？
<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">1 - (4/7)</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">150 / (3/7)</gadget>
<output>350</output>

<result>350</result>