Question

I interviewed 120 people in three social classes

Answer 1

Mate! What I have to do in this?

Answer 2

Answer:

120 people were interviewed in three social classes.

Explanation:

Original Sentence

I [Subject] interviewed [Verb] 120 People [Object] in 3 Social Classes

a passive voice sentence either puts the action (the verb) first and the subject second, or leaves the subject out altogether (impersonal passive)

New Sentence:

120 People were interviewed (past continuous) by me in three social classes.

Note that the answer doesnt include "by me", as passive sentences are more often than not impersonal, to seperate the statement from an entity or person.