A bottle of disinfectant solution weighs 400 grams, and the ratio of medicine to water is 1:4. Some water needs to be added before disinfection and spraying, and the weight of the medicine after adding water accounts for 12.5% of the liquid medicine. How many grams of water are added when using?
某瓶消毒药水重400克，其中药与水的比是1：4．消毒喷洒前还需要加一些水，加水后药的重量占药水的12.5%．使用时加了多少克的水？
<gadget id="calculator">1 + 4</gadget>
<output>5</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">12.5 / 100</gadget>
<output>0.125</output>

<gadget id="calculator">(1/5) / 0.125</gadget>
<output>1.6</output>

<gadget id="calculator">400 * 1.6</gadget>
<output>640</output>

<gadget id="calculator">640 - 400</gadget>
<output>240</output>

<result>240</result>