Fifty per cent of adults add up in the mind (8)
I believe the answer is:
mentally
'in the mind' is the definition.
(I've seen this before)
'fifty per cent of adults add up' is the wordplay.
'fifty per cent of adults' becomes 'men' (50 per cent of the population).
'add up' becomes 'tally' (synonyms).
'men'+'tally'='MENTALLY'
(Other definitions for mentally that I've seen before include "rationally" , "by way of intellect" , "not physically" , "without pencil and paper" , "Intellectually" .)