(c) >I'll finish the work in a couple of days That suggests that the work will not be completed in anything less than a couple of days. At least a few days will pass before the work is completed. >I'll finish the work within a couple of days That suggests that the work might be completed sooner; the worst possible case is that it will take a couple of days to complete.

1. ‘In’ and ‘within’ are both prepositions that refer to spatial or a logical relationship between two or more subjects in a sentence. 2. ‘In’ is specifically defined as being inside, while ‘within’ implies being inside of a given set of parameters relative to the object it’s linking to and not actually inside of it.