Nu mericallv, the variable addressing the cost of the corresponding great would have a negative coefcient in the interest work. For instance, {1d = a P Pg where Q is the amount of cars requested, P is the cost of cars and Pg is the cost of fuel. The other primary class of related products are substitutes. Substitutes are merchandise that can be utilized instead of the essential great. The numerical connection between the cost of the substitute and the interest for the positive qualies being referred to is positive, Assuming the cost of the substitute goes down the interest for the positive qualities being referred to goes down