The vertical demand curve for a commodity shows that this commodity is

✅ Explanation: A vertical demand curve indicates that the quantity demanded does not change at all in response to changes in price. This means that consumers are willing to buy the same quantity of the commodity regardless of its price, indicating perfectly inelastic demand.

📌 🔑Key Points:
-The formula for calculating elasticity is: Price Elasticity of Demand= percent change in quantity/percent change in price
-According to the question price of a good falls from Rs. 20 per unit to Rs. 15 per unit, which means the percentage change in price is 25% and demand rises by 25 percent. 
-By the formula price elasticity of demand= 25%/25%= 1.
Additional Information

✏️ Price elasticity of demand:
-Price elasticity measures the responsiveness of the quantity demanded or supplied of a good to a change in its price.
-It is computed as the percentage change in quantity demanded—or supplied—divided by the percentage change in price.
-Elasticity can be described as elastic—or very responsive—unit elastic, or inelastic—not very responsive.
-Elastic demand or supply curves indicate that the quantity demanded or supplied response to price changes in a greater than proportional manner.
-An inelastic demand or supply curve is one where a given percentage change in price will cause a smaller percentage change in quantity demanded or supplied.
-Unitary elasticity means that a given percentage change in price leads to an equal percentage change in quantity demanded or supplied.