Answer:
Yes there is, and it is 4.99.
Step-by-step explanation:
The constant of proportionality (let's call it [tex]\alpha[/tex]) will relate the number of books bought [tex]N[/tex] and their cost [tex]C[/tex] in the following way:
[tex]C={\alpha}N[/tex]
[tex] \therefore \alpha =\frac{C}{N}[/tex]
For the 12 books:
[tex]\alpha =\frac{59.88}{12} = 4.99.[/tex]
And for the 21 books:
[tex]\alpha =\frac{104.79}{21} = 4.99.[/tex]
The [tex]\alpha[/tex] for the two purchases is the same, thus there is a constant of proportionality between the two purchases.
P.S: [tex]\alpha[/tex] is the price per book.
