Let \(x\) be the amount in liters of the 80% solution. So the amount of the 30% solution is 400 - \(x\).

To obtain 400 L of a 62% solution we need to solve for \(x\) the following equation:

$$ 0.8x + 0.3(400-x) = 0.62 \cdot 400$$

Solving for x gives

$$ 0.8x + 120-0.3x = 248$$

$$ 0.5x= 128 \therefore$$

$$ x= \frac{128}{0.5} = 256$$

Therefore, 256 L of the 80% solution and 400 - 256 = 144 L of the 30% solution should be mixed.