Let the weight of fabric be 1 unit. The dye concentration is given as 0.4 g/L, and the required shade is 2% (which means 2 grams of dye for every 100 grams of fabric).
We can set up the equation based on the material-to-liquor ratio:
\[
\text{Material-to-liquor ratio} = \frac{\text{Weight of fabric}}{\text{Volume of liquor}} = \frac{1}{X}
\]
Now, the amount of dye required for 2% shade is 2 grams for every 100 grams of fabric. To achieve this shade, we need:
\[
\text{Dye concentration} \times \text{Volume of liquor} = \text{Amount of dye required}
0.4 \times X = 2
\]
Solving for \( X \):
\[
X = \frac{2}{0.4} = 5
\]
Therefore, the material-to-liquor ratio is 1 : 50.