An alloy, the weight ratio of copper and tin is 5:7. There are 420 grams of tin, how many grams of copper need to be added to make this alloy.
一种合金，铜与锡的重量比是5：7．现有420克锡，需加入多少克铜才能炼成这种合金．
<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">420 * (5/7)</gadget>
<output>300</output>

<result>300</result>