Some cement was brought to the construction site, 2.5 tons were used on the first day, and the tonnage used on the second day was 2/5 of the first day (2/5). Half of the total cement was used in two days, and there was still leftovers on the construction site. How many tons of cement.
工地上运来一些水泥，第一天用去了2.5吨，第二天用去的吨数是第一天的(2/5)，两天正好用去了全部水泥的一半，工地上还剩下水泥多少吨．
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">2.5 * (2/5)</gadget>
<output>1</output>

<gadget id="calculator">2.5 + 1</gadget>
<output>3.5</output>

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

<gadget id="calculator">3.5 / (1/2)</gadget>
<output>7</output>

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

<gadget id="calculator">7 * (1/2)</gadget>
<output>7/2 = around 3.5</output>

<result>7/2 = around 3.5</result>