If I understand the problem correctly, C.
Original solution was all 50% acid, then x % of that was removed and x % of 30% acid added.
So: (1-x)50+x30=40 solve for x
Or: the average of 50 and 30 is 40, so you need a 50/50 split.
This is a standard problem applicable to mixtures as well as some problems on averages.
What I've shown above is that if A and B are the items whose certain parameter is given as 'a' and 'b' respectively and same parameter of resultant mixture C is given to be 'c',Code:A : B a b c (b~c) : (a~c)
Then the ratio in which A and B have been mixed is (c~b) : (c~a)
['~' sign stands for difference not subtraction]
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