Suppose you'll work it out mathematically (6)

I believe the answer is:

reckon

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'suppose' is the definition.
(I know that suppose can be written as reckon)

I can't explain the remainder of the clue.

(Other definitions for reckon that I've seen before include "what people who count will do" , "Consider, judge to be probable" , "Believe, suppose, judge" , "Think - calculate" , "Calculate or opine" .)

I've seen this clue in the Evening Standard.

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