Altering the numbers in a problem has an effect on the strategies that are preferred.
You can manipulate the numbers to see if students adjust their preferred strategy to suit the problem.
To find out which strategies are best for solving a problem, solve it yourself, then try using other strategies.
Varying the context of the problem also has a pronounced effect on difficulty.
In general, problems that tell of an action are easier to solve than others where the action must be inferred.
For example, difference problems tend to be harder than take away problems. Result unknown problems are usually easier than change or start unknown problems.
Here are some sources of good questions you can use for interviews.
