Now there are three bottles of salt water A, B, and C. It is known that the concentration of A is 40%, the concentration of B is 60%, and the concentration of C is 90%. If A and B are mixed together, the new brine concentration obtained is 50%; if B and C are mixed in Together, the resulting new brine concentration is 70%. Now mix A, B, and C together. Then bring over a new bottle of salt water D. It is found that no matter how much brine D is put into the mixed brine of A, B, and C, the concentration of brine remains unchanged. What is the concentration of salt water D?
现在有A，B，C三瓶盐水．已知A的浓度为40%，B的浓度为60%，C的浓度为90%，如果将A、B混合在一起，那么得到的新的盐水浓度为50%；如果将B、C混合在一起，那么得到的新的盐水浓度为70%，现在将A、B，C都混合在一起．然后拿过来一瓶新的盐水D．发现无论将多少盐水D放入A、B、C的混合盐水中，盐水的浓度都不变．则盐水D的浓度为多少．
<gadget id="calculator">2 / 1</gadget>
<output>2</output>

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

<gadget id="calculator">2 * (2/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">60 / 100</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">2 * (3/5)</gadget>
<output>6/5 = around 1.2</output>

<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 * (9/10)</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">(4/5) + (6/5) + (9/10)</gadget>
<output>29/10 = around 2.9</output>

<gadget id="calculator">2 + 2 + 1</gadget>
<output>5</output>

<gadget id="calculator">(29/10) / 5</gadget>
<output>29/50 = around 0.58</output>

<result>29/50 = around 0.58</result>