1. Convert milligrams to grams:

* 150 mg = 0.15 g (since 1 g = 1000 mg)

2. Calculate the concentration in grams per milliliter:

* Concentration = 0.15 g / 20 ml = 0.0075 g/ml

3. Convert the concentration to a percentage:

* Percentage = (0.0075 g/ml) * 100% = 0.75%

Therefore, the solution contains 0.75% of the active substance.