For a pile of coal, 15 tons are used for the first time, and the rest (2/7) is used for the second time, which is exactly half of the original. How many tons is this pile of coal?
一堆煤，第一次用去15吨，第二次用去余下的(2/7)，剩下的刚好是原来的一半．这堆煤原有多少吨？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">1 - (2/7)</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">(1/2) / (5/7)</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">1 - (7/10)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">15 / (3/10)</gadget>
<output>50</output>

<result>50</result>