No need to brush up your maths. All they are looking for is-
1. If you can distinguish key facts from drivel
2. Have a clear thought process
3. Demonstrate client perspective
4. Can identify problems
The director will probably ask you to use the whiteboard to draw up a mind map of your thought process around the problem and what all related solutions you seem to identify.
Examples of case study might be around two companies merging together and their integration challenges or a new value proposition by telecom operator which aims to change the consumer behaviour and similar situations.
All the best.