The law underlying the given schedule is the Law of Diminishing Marginal Utility. As per the law of diminishing marginal utility, the level of satisfaction obtained from each additional unit of a commodity consumed keeps on declining as more and more units of a commodity are consumed. This law was formulated by the famous economist. 

Alfred Marshall. It is based on the following .assumptions.

• All the units of a commodity consumed must be same in all respect-in size, colour, quality etc. 

• The unit of the good must be the standard unit. 

• No change in taste and preference of the consumer. 

• No time break in consumption of every successive unit of the commodity. 

• No change in the price of a substitute good.

As the schedule illustrates that when the first unit of Pen is consumed, the marginal utility derived is maximum. But with every successive unit pen used, the marginal utility goes on decreasing from 25 utils to 5 utils when 5th unit of pen is used.