#Ratio mixture problem

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weak sun
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depends whether the "ratio" is a ratio of volumes or of weights

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if its of volumes, you just want 1 part for every unit the other thing is away from the desired average

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so, copper is 6 away from the average, so you want 6 parts gold
and gold is 4 away from the average, so you want 4 parts copper

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so you get 3:2

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the procedure, or why it works?

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copper is 9, and you want an average of 15
that is a difference of 6
so you get 6 parts gold

gold is 19, and you want an average of 15
that is a difference of 4
so you get 4 parts copper
6gold:4copper

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the reason it works, is that the further the other element is away from the average, it means the average is closer to you
so you need more of that one

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or, you can you know, do an actual algebraic explanation

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if we have G gold volume and C copper volume, then we want (19G + 9C)/(G+C), that is average density = (total mass)/(total volume) to be 15

19G + 9C = 15G + 15C
4G = 6C
so you can see 6:4 also works here

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because you want the density of the resulting mixture as 15

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well we can assume that, yes

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it does not really mean anything for a mixture to be "15 times as heavy as water"

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as heavy as how much water