Những loại hàng hóa nào khi giá cả tăng thì sức mua cũng tăng và ngược lại. Giải thích lí do

Zachary has been trying to work on an essay for almost three hours, be he can't seem to focus for more than 15 or 20 minutes at a time. what should he do so that the quality of his work doesnt suffer because of poor concentration and lack of energy