Number puzzle that's easy (2,7)
I believe the answer is:
no problem
'that's easy' is the definition.
(if something is no problem it is easy or simple)
'number puzzle' is the wordplay.
'number' becomes 'no.' (abbreviation for 'number').
'puzzle' becomes 'problem' (I've seen this before).
'no'+'problem'='NO PROBLEM'
(Other definitions for no problem that I've seen before include "fine" , "Don't worry about it!" , "of course" , "Easy peasy!" .)