The plural form of the plural noun passengers is passengers'.
Example: All of the passengers' luggage has been stowed.
The plural is relatives. The plural possessive is relatives'.
The plural is lads. The plural possessive is lads'.
The plural form is Mexicans. The plural possessive is Mexicans'.
The plural form is suffixes. The plural possessive is suffixes'.
The plural possessive of stepsister is stepsisters'.
The plural form is echoes. The plural possessive is echoes'.
The plural form is inches. The plural possessive is inches'.
Coaster's is singular possessive. The plural is coasters, the plural possessive is coasters'.
The plural possessive is pals'.
The plural possessive of bandit is bandits'.
Studies' is the plural possessive of study.
The plural possessive form of "weeks" is "weeks'."