Let B be the number of books Bob has.
Since it is said that Jake has twice the number of books Bob has, we can say that Jake has a number of books given by 2B.
Pete has B more books than Bob has, which can be said as B+B, which is also 2B.
For simplicity:
B=Bob books
2B=Jake books
2B=Pete books
Therefore, if we add Bob's, Jake's and Pete's books, we get:
B+2B+2B=5B
Hope that helps.