The concentration of water in the bottle is 25% after adding salt, and the concentration is 15% after adding 400 grams of water. How much water is there in the bottle?
瓶中水加入盐后浓度是25%，再加入400克水浓度为15%，求瓶中原有多少水？
<gadget id="calculator">15 / 100</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">400 * (3/20)</gadget>
<output>60</output>

<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/4) - (3/20)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">60 / (1/10)</gadget>
<output>600</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">600 * (3/4)</gadget>
<output>450</output>

<result>450</result>