Let $n$ be an integer such that $n > 1$. Define the function $f(n)$ as the sum of the divisors of $n$. For example, for $n = 6$, the divisors are 1, 2, 3, and 6, so $f(6) = 1 + 2 + 3 + 6 = 12$. Given the following values:
Quantity A: $f(12)$
Quantity B: $f(15)$
Compute both quantities and determine their relationship.