Final answer:
Mrs. Rodgers should use volume units to calculate the amount of fruit punch she needs.
Step-by-step explanation:
The units that Mrs. Rodgers should use to calculate the amount of fruit punch are volume.
Volume is the measurement of the amount of space an object takes up. In this case, Mrs. Rodgers needs to know how much fruit punch she needs in terms of the container size.
To calculate the volume of the fruit punch, Mrs. Rodgers must consider the number of people attending the reunion. Since 20 people are expected, she should calculate the amount of fruit punch needed based on the serving size per person.