Refer $to$ Exercise $27.$ Let $g(p_1,p_2)be$ the number $of$ people

who will take the train when $p_{1}is$ the price $of$ the bus ride and

$p_{2}is$ the price $of$ the train ride. Would you expect $\frac{\text{d}\text{e}\text{l}\text{g}}{del{p}_1}tobe$

positive $or$ negative? How about $\frac{\text{d}\text{e}\text{l}\text{g}}{del{p}_2}?$