Concrete for construction sites is made by mixing cement, yellow sand, and gravel in a ratio of 1:2:4. Now there are 7 tons of cement, 10 tons of yellow sand, and 20 tons of stones on the construction site. When all the stones are used up, will there be surplus or not enough cement? How many tons are left or not enough?
建筑工地用的混凝土是用水泥、黄沙、石子三种材料按照1：2：4的比例搅拌而成的．现在工地上有水泥7吨、黄沙10吨、石子20吨，当石子全部用完时，水泥是剩余还是不够？剩余或不够多少吨？
<gadget id="calculator">1 + 2 + 4</gadget>
<output>7</output>

<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">20 / (4/7)</gadget>
<output>35</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">35 * (1/7)</gadget>
<output>5</output>

<gadget id="calculator">7 - 5</gadget>
<output>2</output>

<result>2</result>