GMAT PREP : Test Pattern:

There is no specific order - the GMAT works from a question bank organized by difficulty level and types of questions. It will have a target # of questions for SC, CR and RC. Then it will randomly give you a question type - then if you get it right, you might get a more difficult question and can be randomly chosen between SC, CR and RC. If given RC, it will give it to you as a group - so several questions relevant to that passage. Then it will jump to SC/CR until the target # of such questions are reached.

Each GMAT Exam can be slightly different - but the target # of questions for each section will always be reached. The specific algorithm has never been released but it follows these guidelines.