A conversion factor is a fraction or a ratio representing a relationship of two different measurement values. To write 20% m/v to a conversion factor, we need to remember that a percent is a value that represents the amount of a part per 100 units of the whole. M/v in the given value represents that the percentage is by mass per volume. So, to write it as a conversion factor, we do as follows:
20% m/v = 20 mass units / 100 volume units = 1 mass units / 5 volume units
Usually units of this are in g per L. So, it is equivalent to 1 g / 5 L