If Emery took twenty days to read the book, then it took Serena 5*20 = <<20*5=100>>100 days to read the whole book.
The total number of days taken by the two to read the book is 100+20 = <<100+20=120>>120 days.
The average number of days the two take to read the book is 120/2 = <<120/2=60>>60 days.
#### 60