The volume of the 35% solution is 50 L
Known parameter;
The percentage concentration of the salt solution with unknown volume = 35%
The percentage concentration of the 20 liters salt solution = 70%
The percentage concentration of the resulting solution = 45%
Unknown parameter;
The volume of the 35% salt solution
Strategy;
Let x represent the volume of the 35% salt solution that must be mixed to obtain the 45% salt solution
Balance the masses of the salts before and after mixing
The mass of the salts before mixing = 70% × 20 L + 35% × x
The mass of the salt solution mixture after mixing = 45% × (20 L + x)
By conservation principles, we get;
70% × 20 L + 35% × x = 45% × (20 L + x)
0.7 × 20 L + 0.35 × x = 0.45 × (20 L + x)
14 L + 0.35·x = 9 L + 0.45·x
14 L - 9 L = 0.45·x - 0.35·x = 0.1·x
5 L = 0.1·x
x = 5 L/(0.1) = 5 L × 10 = 50 L
The volume of the 35% solution that must be mixed with the 70% salt solution x = 50 L
Learn more about calculations of concentration of mixtures here;
https://brainly.com/question/11866299
