Codes: AC9M10A04. Goal: formulate two linear cost models and say what their terms mean. Prepare: fictional device-hire brief and cost board.
- Context · 2 min. “This is an invented adult household choice. We compare simplified totals; no one will disclose personal finances or make a purchase.”
- Model · 5 min. Let
mbe complete months.A(m)=240+80m: $240 setup at m=0, then $80 per month. Write the parallelB(m)=480+50mwith units. - Guide values · 6 min. Calculate at m=4: A $560, B $680. Ask why multiplying setup by four would be wrong.
- Represent · 7 min. Learners fill a small table for m=0,4,8,10. Use the cost board or text table. Check A/B at 8 both $880 but with different cost structures.
- Assumptions · 3 min. Name one omitted real condition: actual contract, device quality, repair outcome or exit terms. Fixed monthly increase makes these linear within the model; real total cost might not be.
- Exit · 2 min. Everyone explains the 50 and 480 in
B(m)and gives B(4) = $680.
Access: money counters, large text, typed formula, speech/AAC explanation. Do not require a home-budget story from the learner.