"사흘은 사일이 아니에요."
Translation:Three days are not four days.
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사 and 일 are of Sino-Korean origin whereas the equivalent Native Korean words are 넷 and 날. With periods of time when there's words of both Sino-Korean and Native Korean origins, you use Sino-Korean numbers with the Sino-Korean words and Native Korean numbers with Native Korean words.
As for 사흘 you sort of need to just memorize it. But, it's of Native Korean origin so it's based on the Native Korean 셋 rather than the Sino-Korean 사.
The point is teaching the difference between the two words with the same syllable 사. One means 3 and the other means 4. So we have to look at the context to know the difference. When we see the Chinese (Sino-Korean) word 일, we know to use the Chinese (Sino-Korean) number 4, but when we see the Korean expression 사흘, we have memorized that it means 3 days. See the excellent explanation by josueetcom up higher in the comments, and the top comment by Andro0, which is a list we all need to memorize. :)
Why? Go up to the very top comment by Andro0. He wrote a list that we all need to memorize. (My teacher made me memorize it a few years ago.) After you memorize those phrases, AND remember the difference between the Sino-Korean (from Chinese) 일 = day and native Korean 날 = day, this question is much easier. YES, having to learn 2 different number systems is difficult, but you can do it. :)