A kind of brass is made by melting zinc and copper in a ratio of 3:7. How many tons of zinc are needed to produce 100 tons of this brass?
一种黄铜是由锌和铜按3：7熔铸而成的．生产这种黄铜100吨，需要锌多少吨？
<gadget id="calculator">3 + 7</gadget>
<output>10</output>

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

<gadget id="calculator">100 * (3/10)</gadget>
<output>30</output>

<result>30</result>