Question 604184
Let the volume of 10% acid solution is x ml


50 * 50% + x * 10% = (50 + x)*20%
(omit the % sign)
2500 + 10x = (50+x)*20
2500 + 10x = 1000 + 20x
2500 - 1000 = 20x - 10x
1500 = 10x
x = 1500/10 = 150


so, in the end she will have 50+150 = 200 ml of 20% acid solution