for formulas like days outstanding, all you need to know is the turnover version of the formula since it is just 365/turnover ratio.
AR turnover = Sale/Avg. Ar
inventory turnover = cogs/avg. inventory
asset turnover = sales/ avg. total assets
what helped me get these down is think of the name of the formula (AR, inventory, assets), and always put the avg. of that particular asset in the denominator
The numerator will be the Income statement account that closely relates to it (AR and Sales, COGS and Inven, and Assets is just weird with sales lol)
Overall the best way to pass is just grind out multiple choice, but make sure you understand why you got things wrong. I can't stress quality over quantity enough. I only completed about 75% of all questions in my review course (probably about 800 questions attempted).
Auditing essentially is being able to identify the assertions and performing tests to ensure management is actually true to what they are asserting. Ie. Search for unrecorded liabilities test = Designed to test management's assertion of completeness (ensure everything as of year end is recorded and not excluded) etc. etc.
Best of luck!