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Split a utility bill by days each roommate was home

  • For College students
  • You practice Giving the right context
  • Time about 5 minutes

About this exercise

Your apartment splits the $240 October electric bill (in F1) by days each roommate was home, listed in column B. A fifth roommate moves in next month, so the total must cover B2:B6 (row 6 is empty for now). Ask for one formula for C2 in one code block that still works when filled down to C6, then exactly 2 numbered lines labeled Fill down: (why it works in every row, in plain words) and Check: (what Jae owes). Under 45 words outside the code.

Common mistakes when you ask AI for this

  • Not mentioning fill-down. The AI repeats your formula without $ signs, so C3 to C5 break. Try: Add: "It must still work when filled down to C5."
  • Not saying how to split. The AI splits evenly or by some other rule. Try: Add: "Split by days home; total covers B2:B6 for the new roommate."
  • No check. Without a known answer, you can't tell if it's right. Try: Add: "Check: what Jae owes."
  • Getting a menu of options. The AI likes to offer two or three versions of the formula, so you're left to pick and test them yourself. Try: Add: "Give one formula only, in one code block, no alternatives."
  • Getting a tutorial. By default the AI explains absolute references at length. Try: Add: "Each line under 15 words. Nothing else."

What a strong prompt includes

  • Says how to split
  • Mentions filling down
  • Asks for a check
  • Asks for one formula

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