Question 1024233
No need to mention the water part.

A chemist needs 500 milliliters of a 20% acid for a chemistry experiment. The chemist combines X milliliters of a 10% acid and Y milliliters of 30% acid to make the 20% acid. 
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You didn't ask a question.
20% is the average of 10% & 30% --> equal parts.
x = y = 250 ml

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The chemists also needs 500 milliliters of 15% acid. Does the chemist need more of the 10% acid than the 30% acid to make this new mix? Explain.
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Yes, he needs more 10%.
Equal parts gives the average, 20%.
More 10% --> lower than average concentration of the mix.
More 30% --> higher than average concentration of the mix.
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In math terms:
x + y = 500 --> y = 500-x
10x + 30*(500-x) = 500*C where C is the %age of the mix.
15000 - 20x = 500*C
C = 30 - x/25 (0 <= x <= 500)
--> Increasing x decreases C