There are 30 tons of cement on the construction site, (1/2) of the cement is used on the first day, and (2/3) is used on the second day. How much of the cement was used the next day? How many tons of water are used in each of the two days? What fraction of these cements are left useless?
工地上的水泥共有30吨，第一天用去这些水泥的(1/2)，第二天用去的是第一天用去的(2/3)．第二天用去这些水泥的几分之几？两天各用去水多少吨？还剩下这些水泥的几分之几没有用？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 - (1/2) - (1/3)</gadget>
<output>1/6 = around 0.166667</output>

<result>1/6 = around 0.166667</result>