Prorate a mid-month plan upgrade and compute the extra amount due

About this exercise

You're billing a customer who upgraded mid-month. Write a prompt for plain text with lines starting "Old plan days:", "New plan days:", "November charge:", "Already billed:" and "Additional due:", each with the math. Prorate by days in a 30-day November. The new plan starts on Nov 16. Don't mention the free month the rep joked about. Under 80 words. No bullets or numbering.

Common mistakes when you ask AI for this

• Charging the full new price. The AI bills $160 for November and shows $60 additional. Try: Say: "Charge each plan for the days it applies: price x days / 30."
• Mentioning the free month. The AI adds the rep's free month comment. Try: Add: "Do not mention any free month."
• Wrong day counts. The AI counts the upgrade date Nov 12 as the start. Try: Add: "The new plan starts Nov 16: 15 days on each plan."
• Markdown extras. The AI adds bold labels, headings and a closing note. Try: Say: "Plain text, no bold. Output only the lines asked for."
• Trusting the AI to count words. AI is bad at counting and runs past word limits. Try: Ask for fewer words than the real cap and keep each line short.

What a strong prompt includes

• Names the five lines
• Gives the date ranges
• Gives the formula
• Bans the free month
• Caps length

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