Using mixture formula:
Let volume be v, % be p
1 should be the first mixture, 2 should be the second mixture, 3 should be the final mixture.
p₁v₁ + p₂v₂ = p₃v₃
p₁ = 25%, v₁ = 40 p₂ = 60%, v₂ = ? p₃ = 30%, v₃ =?
Let v₂ = x, v₃ = total volumes = v₁ + v₂ = (40 + x)
Using:
p₁v₁ + p₂v₂ = p₃v₃
25*40 + 60*x = 30*(40 + x)
1000 + 60x = 1200 + 30x
60x - 30x = 1200 - 1000
30x = 200
x = 200/30
x = 20/3
Recall x = v₂, so she has to add 20/3 milliters of the 60% solution in order to obtain 30% solution.
I hope this explains it.
