[Numbers]: Rethink/Redesign Second Round

[BethanyG]

We're opening this issue to address problems with our two Numbers concept exercises, Currency Exchange and Electric Bill.

The Challenge: Come up with something better

• The exercise files in the repo: Currency Exchange & Electric Bill
• Refer to this issue for all the details on the problems with Currency Exchange.
• See this PR and this design.md for some detail on redesign choices made for Electric Bill.
• See this forum post for some of the feedback for Electric Bill.
• See this revisions list for the numbers about.md, to get a feel for changes that have been made to the numbers concept over time.

Please use this issue to propose rewrites | restructuring | changes in story to one or the other of these exercises, along with any revisions you'd make to the numbers concept itself. Feel free to link to a PR for these suggestions. Keep in mind the PR will be auto-closed, but we will reopen any PRs that propose solid solutions to our issues. Our ultimate goal is to get either or both exercises to a state where we can all be comfortable / happy with them as numbers concept exercises for the track.

We don't want suggestions on re-working our concept tree or how teach math. We are also very aware of typos, mistakes, and spelling errors. While correcting them is important, that's not what we want here

Instead, we'd like to know what you would do to fulfill the requirements of writing this numbers concept exercise in a way that doesn't raise significant complaints, create confusion for students on the Python track, and is consistent with the intent of concept exercises, and their requirements. Keep in mind that this is the 2nd or 3rd exercise for the track, and many features of Python cannot be used at this stage, including other modules, loops, comprehensions, lists, tuples, sets, and so forth.

We are also not (at this time) interested in copying another tracks exercise, story or concept. We're interested is solving the issues with what we currently have.

Thank you very much for your ideas and support! 🌟