Let a represent the volume of solution A. If it contains 2% of alcohol, then the amount of alcohol that A contains is 2/100 * a = 0.02a
If A = 200, then the amount of alcohol in A is 0.02 * 200 = 4
Let b represent the volume of solution B. If it contains 7% of alcohol, then the amount of alcohol that B contains is 7/100 * a = 0.07b
The total volume of solution A and B is (a + b)
If the volume of A = 200, then the total volume or resulting mixture is 200 + b
If the mixture contains 5% alcohol, the the amount of alcohol in the mixture is
5/100 * (200 + b)
0.05(200 + b)
By equating the concentrations, we have
4 + 0.07b = 0.05(200 + b)
4 + 0.07b = 10 + 0.05b
0.07b - 0.05b = 10 - 4
0.02b = 6
b = 6/0.02
b = 300
She needs to use 300 milliliters of solution B