{"id":"sheet-col-roommate-utility-split-formula","audience":"college","category":"spreadsheets","track":"data","technique":"context","title":"Split a utility bill by days each roommate was home","free":false,"task":"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.","materialLabel":"Your bill-split sheet (attached to your prompt automatically)","material":"Sheet: October bills (Google Sheets)\n\nF1: 240  (October electric bill, $)\n\nA Roommate | B Days home in October | C Owes\nAna | 30 |\nJae | 10 |\nKim | 30 |\nLou | 30 |\n\nNotes:\n- Jae was at a co-op placement for 20 days\n- Last month we split evenly ($60 each) and Jae said that wasn't fair\n- My first formula, =F1*B2/SUM(B2:B5), worked in C2 but C3 showed 0\n- Total of column C should equal the bill","rules":[{"type":"codeBlocks","count":1},{"type":"numberedItems","count":2},{"type":"maxProseWords","max":45}],"mistakes":[{"title":"Not mentioning fill-down","why":"The AI repeats your formula without $ signs, so C3 to C5 break.","fix":"Add: \"It must still work when filled down to C5.\"","checks":["r1"]},{"title":"Not saying how to split","why":"The AI splits evenly or by some other rule.","fix":"Add: \"Split by days home; total covers B2:B6 for the new roommate.\"","checks":["r0"]},{"title":"No check","why":"Without a known answer, you can't tell if it's right.","fix":"Add: \"Check: what Jae owes.\"","checks":["r2","numberedItems"]},{"title":"Getting a menu of options","why":"The AI likes to offer two or three versions of the formula, so you're left to pick and test them yourself.","fix":"Add: \"Give one formula only, in one code block, no alternatives.\"","checks":["codeBlocks"]},{"title":"Getting a tutorial","why":"By default the AI explains absolute references at length.","fix":"Add: \"Each line under 15 words. Nothing else.\"","checks":["maxProseWords"]}],"promptHints":[{"label":"Says how to split","patterns":["days","b2:b6","by days","fifth roommate"]},{"label":"Mentions filling down","patterns":["fill(ed)? down","c6","every row","\\$"]},{"label":"Asks for a check","patterns":["check","jae"]},{"label":"Asks for one formula","patterns":["one formula","one code block","single formula","no alternatives"]}],"hint":"A formula that works in one cell can break when you fill it down. Say you need it to fill down, and ask for one known answer.","rubricLabels":["The formula multiplies the bill by this roommate's days divided by the total of B2:B6 (including the empty row 6, not just B2:B5)","The bill cell and the total range are locked with $ (for example =$F$1*B2/SUM($B$2:$B$6)) so the formula works when filled down","After the code there are 2 numbered lines labeled Fill down: (in plain words, the bill and total stay fixed while the row changes) and Check: (Jae owes $24)"]}