Dr. graham needs 30 l of a 50% acid solution to conduct an experiment. she only has a 60% acid solution and a
20% acid solution in her lab. how many liters of each solution should she mix together?

Let x be the amount in L of the 20% acid and y be the amount in L of the 60% acid. Doing an overall volume balance,
                           V = x  + y = 30
Then, doing an alcohol balance,
                          0.2x + 0.60y = (30)(0.5)
The values of x and y from the generated equations above are
                          x = 7.5L
                           y = 22.5 L
Therefore, she will need 7.5L of 20% acid solution and 22.5L of 60% acid solution.