Question 40867
Let x be the amount of 80% acid.  Thus 400-x is the amount of 30% acid.  Then the setup is
x(.80) + (400-x)(.30) = 400(.62)
.8x + 120 - .3x = 248
.5x = 128
x = 256 liters of 80%
400-x = 144 liters of 30%