Question 183925
.70x+.40*8=.50(8+x)
.70x+3.2=4+.50x
.70x-.50x=4-3.2
.20x=.8 
x=.8/.2
x=4 liters of 70% acid is used.
Proof:
.70*4+.4*8=.5(8+4)
2.8+3.2=.5*12
6=6