fix: improve cash register precision

[gikf]

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

Closes #559

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• Solution updated to version from fix(curriculum): prevent hardcoding cash register freeCodeCamp#55066.
• Added ignored patterns to .eslintrc.json, if needed I can use comment to two, currently, affected lines, or create separate PR.

[lasjorg]

Unlike the current demo code, this code always passes the new last test. I would suggest we merge this to make sure our demo project passes.

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I see no issue with allowing _ as an unused var.