Historical Linguistics and Language Families 4, Exercise 1: which is older?

cantar [k] / chanter [ʃ]: [k] is older. Stops tend to become fricatives, so a
stop turning into a fricative is the natural direction; the reverse is not.
(Worked example from Wikipedia, Linguistic reconstruction.)

Voiceless stop between vowels vs a voiced one: the voiceless stop is the
older one. The source lists "voiceless sounds between vowels voice" as one
of the four most natural developments, so a voiced intervocalic stop is
likely to be the later form.

Caveat: these are rules of thumb. Chapter 3's Romance data and the majority
principle must also agree before a reconstruction is accepted.