After a pile of coal is used (2/5), another 13 tons are brought in, which is (1/4) more than before. How many tons did the original pile of coal weigh?
一堆煤，用去(2/5)后，又运进13吨，这时比原来多(1/4)．原来的这堆煤重多少吨？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 + (1/4)</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">(5/4) - (3/5)</gadget>
<output>13/20 = around 0.65</output>

<gadget id="calculator">13 / (13/20)</gadget>
<output>20</output>

<result>20</result>