Put figures together to sound reasonable (3,2)
I believe the answer is:
add up
I believe this clue is a double definition.
'put figures together' is the first definition.
(to add up is to add figures together)
'sound reasonable' is the second definition.
(to sound reasonable is to add up or make sense)
'to' is the link.
(Other definitions for add up that I've seen before include "Ring true" , "Aggregate, get total" , "Put together (numbers)" , "Seem to be sensible" , "Make sense; count" .)