Xiaomei uses 408 grams of water and 72 grams of salt to prepare a kind of salt water. To make the salt content of this cup of salt water 10%, how many grams of water should be added?
小美用408克水和72克盐配制一种盐水，要使这杯盐水的含盐率变为10%，需要加水多少克？
<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">72 / (1/10)</gadget>
<output>720</output>

<gadget id="calculator">720 - 408 - 72</gadget>
<output>240</output>

<result>240</result>