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Alternating Number Series Puzzle

Alternating Number Series Puzzle Solution - 14 January

Find the missing number in the series below

13  -21    34  -55    ?  -144

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Solution
89

13 -21 34 -55 ? -144
The number is differnce between right number and left number.
i.e

example
13-(-21) = 34
-21-(34) = -55
34-(-55) = 89 answer
-55-(89) = -144