Financial Literacy — Best Buys
Warm-up
Two cereal boxes, two phone photos: 550 g at \$5.79 vs 720 g at \$7.29. Thirty seconds, phones as calculators: which is the better buy? (Unit prices: 1.053 vs 1.013 cents/g — the big box, narrowly.)
Grade 8 financial literacy is comparison shopping at full strength: unit prices with unit conversions, sales stacked with taxes, and the "per-use cost" lens that reframes owning things.
Explore
Best-buy gauntlet: stations with real-world friction — (1) mixed units: 1.36 kg vs 24 oz vs 850 g of the same product (convert EVERYTHING to one unit first); (2) coupons vs sales: "\$2 off" vs "20% off" — which wins depends on the price (crossover at \$10, found algebraically); (3) the multi-buy trap: "3 for \$11" vs \$3.79 each — is the bundle better, and MUST you buy 3? (Read the fine print card.)
Then per-use costing: a \$180 pair of boots worn 200 times vs \$60 boots worn 30 times ($0.90 vs \$2.00 per wear) — the cheap option costs double per use. Durability is a price the sticker doesn't show.
Formalize
Formalize the two comparison instruments:
The fine-print taxonomy that changes answers: "3 for \$11" usually prices EACH at \$3.67 without forcing three (but not always); "buy 2 get 1 free" is a 33% discount ONLY if you wanted three; loyalty prices, member prices, and expiry-driven markdowns all carry conditions. The number is the start; the conditions are the contract.
Practice
Practice: three unit-price battles (one cross-unit); the coupon-vs-percent crossover derivation; one per-use comparison with a defended assumption about "expected uses"; one flyer audit — find the best and worst "deal" on a real flyer page and prove both.
Exit ticket: "\$4 off or 25% off a \$14 item — which, and at what price do they tie?" (\$4 off wins below \$16; tie at \$16.)
Exit ticket
Practice: three unit-price battles (one cross-unit); the coupon-vs-percent crossover derivation; one per-use comparison with a defended assumption about "expected uses"; one flyer audit — find the best and worst "deal" on a real flyer page and prove both.
Exit ticket: "\$4 off or 25% off a \$14 item — which, and at what price do they tie?" (\$4 off wins below \$16; tie at \$16.)
The product: peanut butter — 500 g at \$4.49, 1 kg at \$7.98, 40 oz (1.134 kg) at \$9.49.
Step 1: One unit for all — dollars per kg: A: . B: . C: .
Step 2: Rank: B (\$7.98/kg) < C (\$8.37) < A (\$8.98). The MIDDLE size wins — the giant jar is NOT the best unit price (promo pricing on the 1 kg).
Step 3: The fit check before buying B twice vs C once for a big family: 2 kg at \$15.96 vs 1.134 kg at \$9.49 — depends on consumption before staleness; a jar thrown out at 60% eaten multiplies its true unit price by 1.67.
Step 4: Three lessons in one shelf: convert units first, never assume size wins, and waste is a price multiplier the receipt never shows.
The offers on any item: coupon \$5 off, or 20% off. When does each win?
Step 1: Set them equal to find the tie: .
Step 2: Read the inequality: below \$25, the flat \$5 beats its percent-equivalent (on a \$10 item: \$5 off is 50%!); above \$25, the 20% grows past \$5.
Step 3: Apply: a \$18 book → coupon (\$13 vs \$14.40). A \$70 jacket → percent (\$56 vs \$65).
Step 4: The general instrument: every flat-vs-percent choice has a crossover at — computing it once beats re-deciding per purchase. And the meta-lesson: stores offer BOTH knowing most shoppers grab whichever sounds bigger; the crossover line is the difference between marketing and math.
The candidates: Backpack A — \$35, history says it survives about one school year (≈190 uses). Backpack B — \$95, warrantied and plausibly good for four years (≈760 uses).
Step 1: Per-use: A → ¢/use. B → ¢/use. B is a third cheaper per carry despite costing nearly triple.
Step 2: Stress the assumption — the honest range: if B lasts only two years (380 uses): 25¢/use — now A wins. The verdict FLIPS inside the plausible range, so the durability estimate is the whole decision. State it, source it (warranty, reviews), and decide with eyes open.
Step 3: The cash-flow caveat: per-use favours B, but B needs \$95 TODAY — affordability and optimality are different questions, and "the poor pay more per use" is a real economic phenomenon the class just derived from a backpack.
Step 4: The exit reflex: sticker price ÷ honest lifetime estimate, with the estimate's uncertainty carried into the verdict. That's not just shopping — it's amortization, met at fourteen.