Marginal utility is an important concept to determine how much
a consumer will buy.
Positive marginal utility is when the consumption of additional
item increases the total utility.
Negative marginal utility is when consumption of a additional
item decreases the total utility
Marginal utility can be calculated as change in total utility
divided by change in number of units consumed.
Example:-
Consider a person has four packets of popcorn and he purchases
a fifth packet. Next imagine that a second person has ten packets
of popcorn and purchases one more.
The fifth person buying the fifth packet of popcorn will get
more utility from that fifth packet because of its proportion to
the total.The second person gains far less utility from purchasing
eleventh packet of popcorn because its proportion to the total is
so low.