Explanation:

When the price of a substitute of commodity X falls, the demand for X falls as well. This is because a substitute is a good that can be used in place of another good. When the price of the substitute falls, consumers have an incentive to switch from commodity X to the substitute, as it becomes relatively cheaper.

Reasons why demand for X falls:

1. Substitution Effect: The decrease in the price of the substitute leads to a decrease in the price of using the substitute compared to commodity X. This makes the substitute relatively more attractive to consumers, leading them to switch from X to the substitute. As a result, the demand for X diminishes.

2. Price-Quantity Relationship: According to the law of demand, there is an inverse relationship between the price of a good and the quantity demanded, assuming other factors remain constant. When the price of the substitute falls, consumers perceive it as a better alternative to commodity X, resulting in a decrease in the quantity demanded of X.

3. Substitute Goods: Substitute goods are goods that can be used in place of each other. When the price of a substitute falls, it becomes more affordable and consumers are more likely to choose it over commodity X. This shift in consumer preference reduces the demand for X.

4. Income Effect: When the price of a substitute falls, consumers may experience an increase in their purchasing power. This allows them to allocate more of their income towards the substitute, reducing their demand for commodity X.

Conclusion:

In summary, when the price of a substitute of commodity X falls, the demand for X falls as well. This is due to the substitution effect, the price-quantity relationship, the availability of substitute goods, and the income effect. All these factors contribute to consumers switching from X to the substitute, resulting in a decrease in the demand for X.