A kind of liquid medicine is prepared according to the ratio of medicine and water 2:7. To prepare 27 kg of this liquid medicine, how much water should be added?
一种药水按药和水2：7的比例配制，要配制这种药水27千克，需要加水多少千克？
<gadget id="calculator">2 + 7</gadget>
<output>9</output>

<gadget id="calculator">7 / 9</gadget>
<output>7/9 = around 0.777778</output>

<gadget id="calculator">27 * (7/9)</gadget>
<output>21</output>

<result>21</result>