my educational background-82(10th),74(12th),55(BE) how much to score in CAT to get into IIM A and IIM B?

I am assuming you are from general category. Based on your overall academic profile, you will need to score atleast 99.96 percentile to get a call for the next round.

All the best!

Minimum of 99.5 percentile is mandatory for general category students to get call for further process. They also consider various parameters like work experience, Gender, engineer or non engineer, Accads, extra curricular activities. For preparing and achievement of above mentioned score kindly refer below mentioned books. There are plethora of books available but byfar the best have been mentioned below. 1.How to prepare for Quantitative Aptitude for the CAT by Arun Sharma. 2.Pearson Guide to Quantitative Aptitude for CAT by Nishit Sinha. Moreover, recommended to go through TIME Coaching Institute Material for solved examples and last 5year question sets for Quantitative aptitude. For Easy level preparation you should go with Chandresh Agarwal and notes from CATKing Coaching classes where there is elaborate pattern for beginner.

As per your educational background you have average performance records in your past academics and so you will be required to work more hard for the entrance and later selection rounds.
So what I suggest you is aim scoring above 98 percentile to get into top bschools through CAT and other top management colleges through CAT.

Yes it can be difficult to get in IIM A and B as they are strict with the past academics.
You will have to work harder than the average, focus more, manage and perform better than average.

Work on your profile making. Let it not be average and common.

Stay positive and work towards your goal. There are many other good colleges you can get into if not IIM's but your aim higher can make you reach the stars if not the moon.
But hope, work sincerely and you can make it.

Good Luck.