Batman: Arkham Asylum

There are 240 Riddler Challenges. To unlock this Achievement, you need to solve 204.

Riddler challenges can be solved, for example, by finding the hidden question mark statuettes, scanning for solutions to riddles using the LB button, finding interview tapes, and smashing the chattering teeth that you see scurrying around on the floor.

You can check your progress by pressing the BACK button and toggling over to the Riddler Challenges screen. If you have found a Riddler map for the area you are in, you will see question marks floating around in the vicinity of the solutions on your map.

Riddle Resolver

Gamesradar
http://www.gamesradar.com/batman-arkham-asylum-riddler-guide...

After you unlock 204 out of 240 you will unlock this achievement
Press back button and go in Riddler challenges to see what you are missing.

GL happy hunting