I think GL has, very sensibly, first considered a single unit:
0.4*x + 0.7*(1-x) = 1
-0.3x +0.7 = 1
-0.3x = -0.1
x = 1/3
so, with 30L you simply require (1/3)*30 L
No doubt GL considered the lead up to the final equation too obvious to be worth spelling out!!
Nope! I've just noticed the answer to http://web2.0calc.com/questions/a-10-acid-solution-and-a-70-acid-solution-are-used-to-make-15-liters-of-a-50-acid-solution-how-many-liters-of-the-70-acid-solution-are-u#r2 and GL does use guesswork!!
.