The answer is 60.

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We must look at the results of the sums. They form the following sequence: -6,6,-12,12,-18,18,-24,24,..... So we can say that we have 2 arithmetic progressions: 6,12,18,24,.... and -6,-12,-18,-24,... Taking one of them and appliyng the general term formula of an AP, we have: $a_{n} = a_{1} + (n - 1).r$ ; $180 = 6 + (n - 1).6$ ; $n = 30$ ; But we have to double this result, since we also have the negative numbers. So, the answer is 60.