The ratio of medicine to water in a kind of medicine is 1:200, the existing medicine is 4 kilograms, how many kilograms of water need to be added.
一种药水中药与水的比是1：200，现有药4千克，需加水多少千克．
<gadget id="calculator">4 / 1</gadget>
<output>4</output>

<gadget id="calculator">4 * 200</gadget>
<output>800</output>

<result>800</result>