Given 80 grams of salt water with a concentration of 25%, how many grams of salt should be added to increase the concentration of the salt water to 40%?
现有浓度为25%的盐水80克，要使盐水浓度提高到40%，需要加多少克盐？
<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

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

<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">(3/4) / (3/5)</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">80 * (5/4)</gadget>
<output>100</output>

<gadget id="calculator">100 - 80</gadget>
<output>20</output>

<result>20</result>