If you get called in for the Assessment Centre (AC),
the first AC involves a few tests in the morning: numerical, and written. If you pass, you proceed to the Group Exercise.
You then get called back for the 2nd AC if you pass. This involves a written Case Study. You get put into a room for 100minutes with a laptop and some information. These may be in the form of e-mails, news clippings, etc. You will always certainly receive excel sheets with information on it.
Your task will be to compute a number of values. For instance, how much profit would a company have earned in the next year if they had operated then. The data will not always be 100% accurate, so they expect you to spot the mistakes and correct them.
You will have to write a report as well, and this takes a lot of time. So you will need to throw in your information as you go, and then structure it later. Not all the information is useful, so speed read. Take note of the important bits. A long article may only have 1 or 2 relevant sentences.
You don't need very impressive maths, as long as you can add, subtract, multiply, divide, you will be good. Just be careful of some values (they may be entered as negatives, rather than positives).
Once you have computed everything, report and all, the assessors will print a copy of your report and table, and talk to you about it. They will ask you about mistakes. And point out things with you. Generally, they are trying to understand why you are doing what you did in calculating and doing the report.
Some questions will involve "how did you go about finding the useful information" or something like that. They will also offer a few scenarios to you, and ask you how these particular scenarios may change your results. Think carefully, and listen to the questions properly. It is okay to clarify, but try not to sound nervous or hesitant or thrown off. The questions are simple if you think it step-by-step. Mostly, the questions involve "will this situation increase or decrease the value you have obtained?".
Try to think of things in terms of prices and supply as well. It helps.
You then have an interview with a Senior-Managing Director. It will be very friendly, but professional. They are trying to understand who you are as a person, and what you like about the company, and your understanding of the work that the company does. So be sure you know what the company's work entails, and what you find interesting about it.