After 15% of a pile of cement is used up, another 4 tons are used up, leaving 45% useless. How many tons is this pile of cement originally?
一堆水泥用了15%后，又用去了4吨，还剩下45%没有用．这堆水泥原来有多少吨．
<gadget id="calculator">15 / 100</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">45 / 100</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">1 - (3/20) - (9/20)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">4 / (2/5)</gadget>
<output>10</output>

<result>10</result>