... which is expressed in terms of the units of measurement ED is always 20 PRINCIPLES OF ECONOMICS expressed as a positive number, even though price and quantity demanded move in opposite directions ... million units of food is thousand units of clothing The opportunity cost of a second and third additional million units is 2, 000 and 3,000 units of clothing, respectively b The opportunity cost of ... income of households in metropolitan New York and Washington, D.C., increases 20 % 22 PRINCIPLES OF ECONOMICS b The cost of a train ticket between New York City and Washington, D.C., is reduced...