Hiya!
In terms of where to start, I would suggest familiarising yourself with terms like assets and liabilities, payables and receivables and most importantly EBITDA (earnings before interest tax depreciation and amortisation). I know that the A&O case study in particular involves looking for the EBITDA value and plugging it into a very basic formula that gets given to you. The A&O case study is also the only one I know that asks for an exact figure.
Otherwise at most other firms, I think that they are mostly looking to test your familiarity with basic financial terms and if you can identify any glaring issues. They do not except you to be a financial whiz!
My initial thoughts would be to look at the balance sheet and determine whether the liabilities (which includes any shareholder equity) equal the assets. Reason being that if you're advising the buyer, they evidently do not want to pay too much for a company which has outstanding liabilities. You could rationalise the number you give as needing to include the price of any outstanding liabilities that would need to be settled by the buyer post-acquisition.
Alternatively, cash flows is sometimes a misleading way to value a company because companies in industries like shipping and oil & gas, are typically loss making (especially in the early stages of a project) but hold a lot of valuable assets. Again this is another angle that you can use to rationalise you answer.
The reality of things is that no acquisition value is ever based solely off a company's balance sheet. Valuations are typically based on something called the discounted cash flow analysis (which essentially involves taking the expected cash flows of the company and dividing it by the current value of money). I really doubt you'd ever get asked about DCF in a law firm interview but I'm just including this here for completeness sake.
I hope that helps somewhat. I really struggled trying to understand this kind of thing as well so let me know if you need any more clarification.
