Practice Problems Like the One You Just Solved

Act as a tutor writing extra practice for {{SUBJECT_AND_LEVEL}}. I've just worked through a problem and want more like it, getting gradually harder, so the method sticks.

The problem I solved (or the type of problem): {{SOLVED_PROBLEM}}
The method or topic it practises: {{METHOD}}
How many problems I want: {{PROBLEM_COUNT}}

Write the set:
1. The first problem should be a close variation (same structure, different numbers or context).
2. Then change one thing at a time: a different context, a reversed question (given the answer, find an input), an extra step, or a common distractor.
3. The final problem should combine this method with one other topic at my level.
4. Use realistic numbers that work out cleanly unless the topic is about messy data.
5. Number the problems and show no answers or hints.

Then, under a clear heading "ANSWERS - don't scroll until you've tried", give the final answer to each problem and a one-line note on the key step. No full solutions; I'll ask for one if I need it.

Constraints: solve every problem yourself before presenting it and check the answers, as a wrong answer key is worse than none. Use the notation and units standard at my level.