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, there are 21 tons left. 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)，这时剩下21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">1 - (2/5) - (4/9)</gadget>
<output>7/45 = around 0.155556</output>

<gadget id="calculator">21 / (7/45)</gadget>
<output>135</output>

<result>135</result>