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All answers having "Reckon" as clue:

3 Character Answers:

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SUM



4 Character Answers:

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DEEM

TTLP



5 Character Answers:

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GUESS

OPINE

SPOSE

TALLY



6 Character Answers:

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FIGURE



7 Character Answers:

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COMPUTE

PRESUME

SURMISE

TOTALUP



8 Character Answers:

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ESTIMATE



9 Character Answers:

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CALCULATE