A pile of coal first uses up (2/5) of the total amount, and then uses up (4/9) of the total amount. At this time, the amount used is 31 tons more than the rest. How many tons are there in total in this pile of coal?
一堆煤，先用去总数的(2/5)，又用去总数的(4/9)，这时用去的比剩下的多31吨，这堆煤共有多少吨？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">4 / 9</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">(2/5) + (4/9)</gadget>
<output>38/45 = around 0.844444</output>

<gadget id="calculator">1 - (2/5) - (4/9)</gadget>
<output>7/45 = around 0.155556</output>

<gadget id="calculator">(38/45) - (7/45)</gadget>
<output>31/45 = around 0.688889</output>

<gadget id="calculator">31 / (31/45)</gadget>
<output>45</output>

<result>45</result>