Repeated Items: Used when an item or person's amount is compared multiple times across different groups or ratios. You solve this by finding a common multiple to equate the repeated item. One Item / Total Unchanged: Applied when either only one variable changes while the other remains exactly the same, or there is an internal transfer between variables that leaves the overall total amount unchanged. Difference Unchanged: Used when two different quantities increase or decrease by the exact same amount. This means the numerical difference between them stays constant (often seen in age-related problems). All Items Changed: Deals with more complex scenarios where all variables in the problem change by different amounts. This typically requires setting up simultaneous equations or using the cross-multiplication method on before-and-after ratios.