Answer:
If AB and CD are perpendicular, (slope of AB) x (slope of CD) = -1
9/a = -1
a = -9
1 gallon of milk would be 16 cups :)

Like fractions are fractions with the same <em>denominator</em>. They are not necessary the same value.
Answer:
Is this an arithmetic sequence?
Yes.
Is there a common difference and if so, what is it?
Yes, there is a common difference : 1
The common difference is 1