Success criteria
- I estimate before calculating.
- I align ones, tenths, and hundredths—not the first digits I see.
- I use the estimate to detect an unreasonable exact answer.
Explain
For money, calculate in integer cents: HK$12.95 becomes 1295 cents. This prevents hidden floating-point dust and keeps hundredths exact. Explain why 12.95 + 8.75 cannot reasonably be 100.70.
Independent practice
Estimate HK$12.95 + HK$8.75 + HK$4.30 by rounding each item to the nearest dollar.
Find the exact basket total.
You pay HK$50.00. How much change should you receive?
Which habit best catches a misplaced decimal point?
An estimate gives a sensible range for the exact result.
Transfer — Checkout Champion (offline)
Use three receipt or basket cards from home. Before exact calculation, estimate each item to the nearest dollar. Annotate one incorrect total: show the place-value mistake and correct it. This is a paper/real-world task, not a coded game.
Delayed check
One week later, total HK$18.40 + HK$6.75 + HK$2.99, then find change from HK$50. Estimate first.
Evidence and next step
Save a photo or scan at jeremy/portfolio/math/unit-1/checkout-champion.jpg and link the one-week check beside it.
- Exact, explained, and retained → continue to decimal multiplication/division.
- Accurate but no estimate → repeat one receipt with estimate-first.
- Place-value or change error → repair with cents, then translate back to dollars.