Final answer:
A decrease in the price of hamburgers increases their quantity demanded, which likely increases the demand for complementary goods like french fries. The demand for hamburgers is uncertain when the price of substitutes (hot dogs) increases and the price of complements (hamburger buns) increases simultaneously.
Step-by-step explanation:
When the price of hamburgers decreases, according to the law of demand, the quantity demanded of hamburgers will increase because consumers are more likely to purchase more hamburgers at a lower price. Furthermore, if hamburgers and french fries are typically consumed together, french fries are considered a complementary good to hamburgers. Therefore, an increase in the quantity demanded of hamburgers is likely to increase the demand for french fries as they are often consumed together.
In the context of the second scenario, the effects on the demand for hamburgers when the price of a substitute good (like hot dogs) increases while the price of a complement good (like hamburger buns) also increases is less clear. The demand for hamburgers might increase because the higher price for hot dogs could cause consumers to switch from hot dogs to hamburgers. Conversely, the demand for hamburgers might decrease because the higher price for hamburger buns makes hamburgers more expensive to make or buy as a whole. Predicting the exact outcome would require more information about consumer preferences and the magnitude of the price changes in the substitute and complement goods.