Constant of proportionality is the constant ratio in a proportional linear relationship. For example, when one lemon costs 0.25, two lemons cost 0.50. What is the unit price per lemon?

• A. 25 cents per lemon
• B. 50 cents per lemon
• C. 12.5 cents per lemon
• D. 75 cents per lemon

Answer: (A)

This tests understanding of unit price—the constant amount charged per single unit in a proportional relationship between quantity and total cost. Since the price scales directly with the number of lemons, the cost must stay at the same amount per lemon. If one lemon costs 0.25 dollars, then each additional lemon adds another 0.25 dollars, so the unit price is 0.25 dollars, i.e., 25 cents per lemon. The fact that two lemons cost 0.50 dollars confirms this: 2 × 0.25 = 0.50. Any other value would not match the given price for one lemon.