20 tons of cement were prepared before the start of a project, and (1/5) was left at the end of the project. How many tons of cement were used in this project?
一项工程开工前准备了20吨水泥，工程结束时还剩下(1/5)，这项工程用了多少吨水泥？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">20 * (4/5)</gadget>
<output>16</output>

<result>16</result>