YA fiction: Boy checks out a Book in his school library. Inside is an envelope with Cash and a Quest. Each time he solves the Riddle, he gets another envelope with cash and a quest. Girl helps him. Read around late 1990s/early 2000s. Spoiler.

[Courtney]

He has to figure out the riddle on the paper. Each time he does he gets another envelope with more cash. A girl from his school joins him and they split the money. If I recall correctly everything came to some science place in their town at the end.
I think I read this in the late 90's early 00's.

[Kris]

Courtney, I copied a few plot details to the header/ topic title. Feel free to edit it.

Location/ country?

[Tab]

What sort of quest did the boy go on?
Part of a series?
Anything about the cover?

[Courtney]

Tab wrote: "What sort of quest did the boy go on?
Part of a series?
Anything about the cover?"

It had to do with the Science Musuem in their town
Nope
I don't remember anything