First of all, the second statement tells us nothing as 35% means 7:13 or 35%:65% ratio.

From the first statement we can find how much water and acid we have in 50g of 35% solution and it's sufficient. If it's not obvious, we can use following calculations:

acid = 0.35x

water in the solution: 0.65x

% of new solution = 0.1 = 0.35x / (w + x)

0.1*(w + x) = 0.35x - meaning: acid in new and old solutions (it should be the same number)

so, here is your mistake: 0.35x

+ w = 0.1(x + w)

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