A potion is prepared by mixing medicine and water at a ratio of 1:20. How many kilograms of medicine are needed to prepare 84 kg of this potion.
一种药水由药和水按1：20配制而成，要配制这种药水84千克需要药多少千克．
<gadget id="calculator">1 + 20</gadget>
<output>21</output>

<gadget id="calculator">1 / 21</gadget>
<output>1/21 = around 0.047619</output>

<gadget id="calculator">84 * (1/21)</gadget>
<output>4</output>

<result>4</result>