When you enter a fast food restaurant, you are presented with a variety of options. Since you are hungry you choose to order food. On the menu are Tacos, Burritos, Nachos, and Sodas. You only have enough money for three items. Since you do not enjoy the aroma of cheese you avoid the nachos; and since you know that you'll be thirsty you choose the soda. So you order one taco, one burrito, and one soda. You had the opportunity to order nachos but you could not afford the cost. The nachos then become the opportunity cost.
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If we were to add several more options to our scenarios above, we could then apply the concept of thinking at the margin. If the fast food restaurant had small, medium, large, and extra large varieties on all food and drinks, then you may consider decreasing or increasing one size option so as to similarly increase or decrease the size of another option. Thinking at the margin occurs when a choice is slightly modified by a factor of one to affect a change on the other possible options. If your boss sent you home one hour early to avoid paying you additional money, then he or she is thinking at the margin by reducing their labor cost by one unit of time.