(a) dvd price $90 , valve $ 150
(b) a t-shirt price $90,value $120
(c)a set dinner price$90,value $ 100
你有$90,如果揀左DVD,the cost is the t-shirt or its value $120 ...如果the price of the set dinner 係 $130...咁揀DVD既cost係咩????? 究竟cost係視乎price 定係value?????請解釋