Question 188891
1 liter of water changed the concentration from 40% to 30%. 
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0.4*x = 0.3*(x+1)
4x = 3x + 3
x = 3 liters or 40%
4 liters of 30%
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To check:
40% of 3 l = 1.2 l of acid
30% of 4 l = 1.2 l of acid