Answer:
1.2 litres of the 30% solution.
Explanation:
0.30x + 0.80y = 0.70(x + y)
0.30x - 0.70x + 0.80y - 0.70y = 0
-0.40x + 0.10y = 0
Also
x + y = 6
Multiply the third equation by 10:
-4x + y = 0
Subtracting the last 2 equations:
5x = 6
x = 6/5 = 1.2 litres.