Let's use the decimal equivalent of percent. He has 50 ml of a 0.15 acid solution. He wants the concentration of acid to increase to 0.25 by adding pure acid.
concentration = (volume of acid)/(total volume of solution)
He takes the current solution and add x amount of pure acid.
15% of 50 ml = 7.5 ml
The current 15% solution contains 7.5 ml of pure acid. The x amount of acid he adds is added onto the 7.5 ml of acid he already has and it is also added to the total volume of 50 ml.
(7.5 + x)/(50 + x) = 0.25
7.5 + x = 0.25(50 + x)
7.5 + x = 12.5 + 0.25x
0.75x = 5
x = 6.6666...
Answer: 6 2/3 ml