Final answer:
A decrease in the price of a normal good leads to an increase in quantity demanded due to both substitution and income effects. The substitution effect causes consumers to buy more of the cheaper good in place of more expensive alternatives, while the income effect increases consumers' purchasing power, allowing them to buy more overall.
Step-by-step explanation:
When a normal good decreases in price, both the substitution effect and the income effect play a role in consumer behavior. The substitution effect indicates that the consumer will tend to buy more of the now cheaper product and less of other more expensive substitutes. This is because the product has become cheaper relative to other goods. On the other hand, the income effect signifies that the consumer's purchasing power increases as the price of the good falls. Consequently, the consumer is able to buy the same amount of goods as before but with some money left over, which could be used to buy more goods or save. Together, these effects typically lead to an increase in the quantity demanded of the lower-priced good.