The construction site mixes cement, sand and stones in a ratio of 2:3:5 to make a kind of concrete. How many tons of sand are needed to make 40 tons of this concrete?
建筑工地把水泥、沙子和石子按2：3：5的比例配制成一种混凝土．要配制40吨这样的混凝土，需要沙子多少吨？
<gadget id="calculator">2 + 3 + 5</gadget>
<output>10</output>

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

<gadget id="calculator">40 * (3/10)</gadget>
<output>12</output>

<result>12</result>