We need 1,600 ml of 70% ethanol.
x is ml of 60% and 
y is ml of 75%
A)  x + y = 1,600
B)  .60x + .75y = .70 * 1,600
Multiplying equation A by -.60
A) -.60 x -.60y = -960 Then adding this to equation B
B)  .60x + .75y = .70 * 1,600 
.15y = -960 + 1,120
.15y = 160
	
	
		
			y  (75%) = 1,066.66 ml 
x (60%) = 
	
	
		
			533.33 ml