Limit problem

$\large \lim_{n\to\infty} \dfrac{1^a + 2^a + 3^a + \cdots + n^a}{(n+1)^{a-1} \left [(na+1) + (na + 2) + \cdots + (na+n) \right ]}$

Find the sum of all constant values of $a$ such that the limit above is equal to $\dfrac1{60}$ .