Sum modulo 10 - Part 1

Number Theory Level pending

If \(a_n\) = \( \frac {n \cdot \overline{n+1}}{2} \pmod{10} \), then find the value of \(k\) so that \(a_1 + a_2 + a_3 + a_4 + \ldots + a_k = 2015\).

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