The construction site needs to mix 15 tons of concrete. The ratio of cement, gravel and sand is 3:3:4. How many tons of cement do you need?
建筑工地要搅拌混凝土15吨，水泥、石子和沙的比是3：3：4，需要准备多少吨水泥？
<gadget id="calculator">3 + 3 + 4</gadget>
<output>10</output>

<gadget id="calculator">3 / 10</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">15 * (3/10)</gadget>
<output>9/2 = around 4.5</output>

<result>9/2 = around 4.5</result>