We know that the value of the discount is equal to $4 and that it represents 25% of the original value. To find the regular price we can apply a rule of three in such a way that $4 is related to 25% in the same proportion that "x" is related to 100%, where "x" is the regular price of the book. We have:
[tex]\frac{4}{x}=\frac{25}{100}[/tex]Doing the cross product we have:
[tex]\begin{gathered} 25\cdot x=4\cdot100 \\ x=\frac{4\cdot100}{25}=16 \end{gathered}[/tex]The regular price of the book is $16.
