Let's begin by listing out the given information:
Before Christmas: 15 books
After Christmas: 21 books
This is an increase
The percentage increase is given by % increase = Increase ÷ Original Number × 100:
[tex]\begin{gathered} \text{\%}increase=Increase\div OriginalNumber\times100\text{\%} \\ \text{\%}increase=\frac{21-15}{15}\times100\text{\%} \\ \text{\%}increase=\frac{6}{15}\times100\text{\%}=40\text{\%} \\ \text{\%}increase=40\text{\%} \end{gathered}[/tex]