The number 21 refers to the number of people out of 100 who used to read books every night and obtained a B in their classes.
This chart shows the number of people, their grades and whether they used to read books.
The number 21 is aligned with the column read books nightly and the row B average. This means this number implies that 21 out of the 100 people read books nightly and obtained a B in their classes.
Note: This question is incomplete because the chart is not provided. Below, I attached the figure.
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