x = amount (in liters) of the 10% solution she should use
y = amount of the 30% solution she should use
She wants a 30L solution, so
x + y = 30
Each liter of the 10% solution contributes 0.1x liters of acid, and each liter of the 30% solution contributes 0.3y liters of acid. 30L of 25% acid solution contains 30L*0.25 = 7.5L of acid, so
0.1x + 0.3y = 7.5
Now solve for x and y:
x + y = 30 ==> y = 30 - x
0.1x + 0.3y = 7.5 ==> 0.1x + 0.3(30 - x) = 7.5
... ==> 0.1x + 9 - 0.3x = 7.5
... ==> 1.5 = 0.2x
... ==> x = 7.5 ==> y = 22.5
