Sreelatha,
Here are my answers for you:
As we are PMP certified I believe we need 21 hours of PDU's? Correct me if I am wrong.
This is partially correct. You need 21 CONTACT HOURS of training in Agile Practices. So in other words you must have participated in an Agile Training that was at least 21 hours long. (And it's OK if the certificate says '21 PDUs' -- as long as it was an "Agile" course). Please review the requirements in the PMI-ACP handbook on page 6.
am preparing for PMI ACP. I am going through Mike Griffith book. Will this be sufficient for the exam?
Many people are using this book but the book alone is not sufficient. You need to study, get the 21 hours of training and you should also use an exam simulator to prepare. Read some lessons learned from other PMI-ACP students and prepare a study plan based on their recommendations.
How much should be the score out of 100 questions as (out of 120 , 20 questions are pretest) to clear the exam?
PMI does not disclose what score you need to pass the exam. Just like with the PMP exam this is based on "psychometric analysis" and PMI does not release any further details.
