Answer: 3 million.
Step-by-step explanation:
Unemployment is defined as when a member of a Country's labor force is jobless but actively looking for work.
In the Scenario 28-1, the discouraged people are not counted as they are discouraged and not looking for work and 1 million other people being students and retirees amongst others are not looking for work either.
The unemployed section of Mankiwland is therefore the 3 million unemployed people.