Makes sense to the mathematician (4,2)

I believe the answer is:

adds up

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'makes sense' is the definition.
('add up' can be a synonym of 'make sense')

I can't explain the remainder of the clue.

(Other definitions for adds up that I've seen before include "Totals" , "Puts two and two together" , "Reckons" , "Stands to reason" , "Tots" .)

I've seen this clue in the Evening Standard.

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