The first step in solving this problem is to compute for the original rate of Aphrodite.
144 leagues / 12 days = 12 leagues/day (original rate)
It was stated in the problem that she doubled her rate which she means she has a new rate of 24 leagues/day (12 x 2).
168 leagues / 24 leagues/day = 7
Therefore, it would take her 7 days to travel 168 leagues at her new speed.