Final answer:
Karin read 1/5 of the book on the first day.
Step-by-step explanation:
Karin read a book in 3 days. On the first day, he read 1/5 of the book.
To find out how much Karin read on the first day, we divide the book into 5 equal parts (since 1/5 means one out of five parts).
So, Karin read 1 part of the book on the first day.