At a price of $15 each, marta buys 4 books per month. when the price increases to $20, marta buys 3 books per month. luz says that marta's demand for books has decreased. is luz correct?

No, Luz is incorrect. Marta's quantity demanded has decreased, but her demand has stayed the same. It is true that As a price for a product increases, the demand for that product will be most likely to decrease because consumers have to make more sacrifice without any additional income.

But, if you pay attention to the case above, you can see the total value of demand that marta has is still the same, which is $ 60