Now there is brine with a salt content of 30%, and 12 grams of salt is added to it. To keep the salt content constant, how many grams of water need to be added.
现在有含盐率30%的盐水，往里面添加12克盐，要保持含盐率不变，则需要加水多少克．
<gadget id="calculator">12 / 30</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">70 * (2/5)</gadget>
<output>28</output>

<result>28</result>