There are 30 tons of yellow sand on the construction site, 20% of the total amount is used for the first time, and 1/2 of the total amount is used for the second time, how many tons of yellow sand are left?
工地有30吨黄沙，第一次用去总数的20%，第二次用去总数的(1/2)，还剩多少吨黄沙？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 - (1/5) - (1/2)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">30 * (3/10)</gadget>
<output>9</output>

<result>9</result>