What is the meaning of 'part of a whole'?
Sometimes the variables we are analyzing naturally represent a whole. For example, we might ask 500 people if they agree with a statement. They'd have to choose one answer on a Likert-type scale: Strongly Disagree, Disagree, Neutral, Agree, or Strongly Agree. These answers (Agree, Disagree, etc.) are separable options. One person can't choose two answers; they have to choose just one. So in this type of problem, it's important to note that the individual parts of a whole can't overlap each other.
These individual parts together compose the whole; after totaling up how often each answer was chosen, we can see how it fits as part of the whole. If we convert these numbers to percentages and add them up, they will total 100%.
If we want to find out how parts contribute to the whole, we have a part of a whole problem, which is also called a composition problem.