Stones, sand and cement are needed to prepare concrete, and their mass ratio is 5:3:2. To prepare 20 tons of concrete at a time, how many tons of sand should be purchased.
配制混凝土需用石子、沙子和水泥，它们的质量比是5：3：2，一次要配制20吨混凝土，需购进沙子多少吨．
<gadget id="calculator">5 + 3 + 2</gadget>
<output>10</output>

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

<gadget id="calculator">20 * (3/10)</gadget>
<output>6</output>

<result>6</result>