Emery and Serena go to their school library, and each borrows a copy of The life of Jack Steve's book to read for their school project. If Emery can read five times as fast as Serena, and the book takes her 20 days to read, what's the average number of days the two take to read the book? Let's think first. Chain of thought:
If Emery took twenty days to read the book, then it took Serena 5 * 20 = 100 days to read the whole book. The total number of days taken by the two to read the book is 100 + 20 = 120 days. The average number of days the two take to read the book is 120 / 2 = 60 days.
Therefore, the answer is 60.