-57
explained in http://wiki.answers.com/Q/Next_number_in_series_298_209_129_58_-4
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At each step the number being subtracted is reduced by 9 298 - 89 = 209 209 - 80 = 129 129 - 71 = 58 58 - 62 = -4 -4 - 53 = -57..........which is the next number.
-57 298 - 89 = 209 209 - 80 = 129 129 - 71 = 58 58 - 62 = -4 -4 - 53 = -57 notice every number is subtracted by 9 less than the previous number's subtrator
-57
-57
-57. You are taking away 9 less each time.