Alexis and Kylie started reading their library books at 7:15 p.m. Alexis read her book for an hour. Kylie read her book 20 minutes less than Alexis.
If Alexis read her book for an hour, then she finished at 8:15 p.m. Kylie read her book for 40 minutes (20 minutes less than Alexis), so she finished at 7:55 p.m.
Therefore, Kylie finished reading her book earlier than Alexis.