There are two vessels containing 40 kg and 30 kg of acid solution of various concentrations.

There are two vessels containing 40 kg and 30 kg of acid solution of various concentrations. If we merge them together, we get a solution containing 73% acid. If equal masses of these solutions are merged, the resulting solution will contain 74% acid. How many kilograms of acid is in the second solution?

Let x be the amount of acid in the first solution and y in the second. Let’s compose a system of equations:
40x + 30y = 0.73 * (40 + 30)
40x + 40y = 0.74 * (40 + 40)
Let’s open the brackets, change the signs in the second equation.
40x + 30y = 51.1
-40x-40y = -59.2
Let’s solve the system using addition:
10y = 8.1
y = 0.81
Answer: 81