1. 12 시에 도착할 수 있어요? = Can you arrive at 12 o'clock? 2. 12 시까지 도착할 수 있어요? = Can you arrive by 12 o'clock? The book's answer is correct. Even though a specific time is given, "by 12 o'clock" doesn't mean exactly at that time. It means anytime before 12 o'clock, and that is exactly what -까지 means in such contexts. 에 corresponds to "at", 까지 to "by", and 전에/후에 to "before/after" (12시 전에 = before 12; 12시 후에 = after 12). One difference in Korean is that 까지 is also used for "until". There's no separate word for "until". - 나는 12시까지 일을 계속했다 = I continued to work until 12 o'clock.