If Dionte creates a list, it will be (3*n, 9*n) where "n" is the number of books, then, we have the pattern:
But how do discover which one should be on the list? See that the first coordinate is divisible by 3, if we divided it by 3 we have just "n", if we do divide the second coordinate by 9 we have just "n" as well. Then we have to divide by three the first coordinate and by 9 the second one, after that they must be the same! if they are the same it's correct.
a)
Let's do our logic, divide by 3 and the other by 9
As we can see it's different, so it couldn't be on Dionte's list.
b)
Doing the same thing
The result is (4,4), two equal numbers, then it could be on Dionte's list.
c)
The result is (5,5), two equal numbers, then it could be on Dionte's list.
d)
The result is (6,6), two equal numbers, then it could be on Dionte's list.
e)
Here we have a different result again, then it couldn't be on Dionte's list.