Lesson 8: Buy Now, Pay Later: What Does It Really Cost?

Prepared by Georgia Wang

Year level: Year 10 (middle ability set)Duration: 60 minutesFocal concept/topic: Buy Now, Pay Later: What Does It Really Cost?

Lesson aim:

Investigate stated BNPL terms and explain how charges change total purchase cost.

Learning intention:

We are learning to calculate BNPL purchase costs under clearly stated payment conditions.

Success criteria:

□ I can identify payment amounts, dates and separately stated charges.
□ I can calculate on-time and specified late-payment totals without double counting.
□ I can test a cost calculation in a spreadsheet and explain it.
□ I can explain why “interest-free” alone does not establish that an offer has no fees.

Syllabus outcomes and content:

NSW Education Standards Authority (NESA, 2022).

MA5-FIN-C-01: solves financial problems involving simple interest, earning money and spending money
MAO-WM-01: develops understanding and fluency in mathematics through exploring and connecting mathematical concepts, choosing and applying mathematical techniques to solve problems, and communicating their thinking and reasoning coherently and clearly
Examine payment options involving buy now, pay later and investigate the costs associated with these schemes for purchasing goods

Working Mathematically indicators:

Connect a payment schedule with a total
Test a model against hand calculations
Explain conditional costs and missing information

ICT:

Pairs build and audit spreadsheet formulas, then vary an explicitly stated charge to investigate changing totals. One student enters; the other checks against handwritten arithmetic; swap roles.

LIT:

Annotate “including”, “additional”, “on time” and “late”
Compare a headline with complete terms
Evidence sentence frame

Prerequisite knowledge:

Lesson 7 deposit–repayments–total process
Percentages
Calculator use
Basic spreadsheet cell entry and multiplication/addition

Resources:

Printed Tasks 8A–8F
Calculators
Mini-whiteboards
Paired devices with spreadsheet
Blank cost-comparison table
Paper equivalent

Teacher model: Spending_Money_Teacher_Spreadsheet.xlsx, L8 BNPL sheet. A completed formula-and-values snapshot follows below. Students construct their own model before seeing the teacher version.

Labels used in this plan: WM Working Mathematically LIT Literacy ICT Technology AfL Assessment for learning AoL Assessment of learning

Key teacher question → Expected: suggested student response. Self-designed WM indicators are in italics.

Teaching sequence

Steps & timing	Teaching & learning activities
ORIENTATION0–8 minRetrieval and bridgeAfL	Teacher: Review a common Lesson 7 error without naming students. Use Task 8A. Students: Calculate independently and explain why the first payment is not added twice. AfL If duplicated, highlight that four payments includes the payment today; introduce LI/SC. Key question: Does “four payments including today” mean four payments or five? Expected: Four. Today is payment 1; $400 ÷ 4 = $100 each, total $400.
ENHANCING8–18 minWorked scheduleLITWM	Teacher: Model Task 8B using Day 0, 14, 28, 42. Read all conditions before computing. Students: Annotate on-time and late scenarios separately. LIT WM Ask “Does no interest mean no charges?” Expected: not necessarily; inspect the stated terms. Key question: Why is the on-time total for the headphones $492 rather than $480? Expected: Four $120 payments plus four separate $3 account charges: $480 + $12 = $492.
ENHANCING18–23 minHinge and responseAfL	Use Task 8C independently. AfL At least 80% correct with sound working: proceed. Otherwise model the omitted account charge or duplicated principal, then ask the fresh check in 8C. Key question: What does “$2 at each payment date” change in the hinge calculation? Expected: Three dates create $6 in charges; total $306, answer C.
ENHANCING23–40 minICT investigationICTWMAfL	Teacher: Give Task 8D. Demonstrate input headings, not completed answers. Students: Hand-calculate first; build formulas in pairs; swap roles; test one altered late-fee input. ICT WM AfL Compare formula output with arithmetic; ask what changes and what remains fixed. Paper fallback uses identical input columns. Key question: Which spreadsheet quantities are recurring and which are already totals? Expected: Payment amount × count is recurring; the account-charge column is already a total. Late fee × late-payment count is added separately.
ENHANCING40–50 minCritical comparisonLITWM	Students: Answer Task 8E individually, then discuss in pairs. LIT WM Distinguish lower cost, schedule fit and unknown affordability. Teacher: Ask for calculations and remaining unknowns, not a universal product recommendation. Key question: Which offer fits the buyer limits, and is that also the cheapest offer? Expected: B fits: $94 today, $80 later. A costs $14 less on time, but requires $160 payments. Schedule fit and total cost differ.
SYNTHESIZING50–55 minExplain and reviseAfL	Students: Correct “Offer B is interest-free so its total is always the cash price.” Expected: on time it costs $654 because of account charges; the specified late case costs $669. AfL Listen for conditional language. Key question: What happens if the late fee changes but there are no late payments? Expected: Nothing changes in total paid: the fee is multiplied by zero. Interest-free does not by itself rule out other charges.
SYNTHESIZING55–60 minIndependent exitAoLAfL	Students: Complete Task 8F; no partner or spreadsheet help, calculator allowed. AoL End-of-lesson individual mastery record against the success criteria. Mark the exit task out of 4 using the criteria below; this records attainment after instruction. AfL Sort errors into payment count, omitted charge and interpretation; use them for Lesson 9 retrieval. Contingency: Reduce sharing, preserve exit.

Worked solutions

8A: A $400 item is paid in four equal payments, including one today, with no charges. Each = $100; total = 4 × $100 = $400, not $500.

8B: $480 headphones. Four payments of $120 on Days 0, 14, 28 and 42. A separate $3 account charge is paid at each scheduled payment date. If exactly one payment is late, an additional $10 charge applies once. On time: 4 × 120 + 4 × 3 = $492; additional cost $12. Specified late case: $492 + $10 = $502; additional cost $22. The $10 does not replace the late $120 payment.

8C Hinge: $300 item; three payments of $100, including today; $2 charge at each date; no late fee in this scenario. A $300, B $302, C $306, D $406. Correct C = 3 × 100 + 3 × 2. Fresh check: $200 item, four $50 payments with $1 charge each → $204.

8D: $640 tablet. Cash $640. Offer A: four $160 payments including today, no account charge, $8 per late payment. Offer B: eight $80 payments including today, $14 account charge in total, $15 per late payment. Calculate on-time total and total with exactly one late payment.

Option	On time	One late payment	Additional cost: on time / late
Cash	$640	Not applicable	$0 / not applicable
A	$640	$648	$0 / $8
B	$654	$669	$14 / $29

Student-built spreadsheet: choose a clear layout with columns for Offer, Payment amount, Payment count, Total account charges, Fee per late payment, Late-payment count and Total paid. The completed teacher file uses inputs in rows 6 and 7: G6 = B6*C6+D6+E6*F6, copied to G7. Set F6 and F7 to 1 for one late payment. Increase E6 to $12 with F6 = 1: total $652; with F6 = 0 it remains $640. Account charges in D are totals. Students may use other row positions if their references remain consistent.

8E: Both offers require the first scheduled payment today; remaining A payments are fortnightly, remaining B payments weekly. Fictional buyer can pay no more than $100 today and $90 at each later date; B’s $14 account charge is payable separately with its first payment. Which fits these limits? B: $94 today and $80 later. A fails: $160 today and later. Cash fails today: $640. A is cheaper on time by $14. These ceilings alone do not establish affordability or credit eligibility. Ask what income timing and other commitments are missing.

8F Exit: $360 item; six payments of $60, including today; separate $2 charge at each date; one additional $9 fee for exactly one late payment. On time = $360 + $12 = $372. Specified late case = $381. Explain “interest-free means no additional cost”: false in this offer; charges add $12 or $21.

Exit assessment criteria

Task 8F: on-time total $372 with payment and charge count shown (1); one-late total $381 (1); interest-free does not mean fee-free (1); cites $12 or $21 additional charges under the correct scenario (1). AoL records four criteria separately. Use the same evidence formatively to decide next-lesson feedback. A correct total without an explanation does not demonstrate the reasoning criterion.

Completed teacher spreadsheet example

File: Spending_Money_Teacher_Spreadsheet.xlsx, L8 BNPL. Offer inputs follow Task 8D. Columns A–F are editable; G calculates total paid. Row 6 contains A, 160, 4, 0, 8, 0; row 7 contains B, 80, 8, 14, 15, 0. G6 = B6*C6+D6+E6*F6; G7 uses the same pattern. On-time outputs: $640 and $654. Set late-payment counts to 1: $648 and $669. Set A late fee to $12 with one late payment: $652. A separate comparison block displays both on-time and one-late totals without changing the input scenario.

Adjustments and anticipated misconceptions

Barrier / misconception	Specific support and retained demand
Adds payment today twice	Mark today as payment 1 of the stated total number.
Omits charges / repeats total charge	Underline whether a charge is per payment or a total; place it in the appropriate column.
Spreadsheet difficulty	Give headings and cell locations; retain student-built formula and handwritten check.
EAL/D / reluctant speaker	Annotate short terms and use “Under the __ scenario, total cost is __ because …”.
Extension	For B on-time to equal A with one late payment, A’s late fee must be $14; justify 640+f=654.

Vocabulary

Term	Student-friendly definition
BNPL	A payment arrangement that divides a purchase into scheduled payments.
Interest-free	No interest is charged under the stated conditions; other charges may still apply.
Account charge	A separately stated charge for the payment arrangement.
Late fee	An additional charge when a specified payment is late.
Payment schedule	The dates and amounts of required payments.
Conditional total	Total for a stated scenario, such as exactly one late payment.

Sources acknowledged in this lesson

NSW Education Standards Authority. (2022). Mathematics K–10 syllabus: Stage 5, Financial mathematics A. https://curriculum.nsw.edu.au/learning-areas/mathematics/mathematics-k-10-2022/content/stage-5/facb3aa952

Syllabus content and outcome descriptions above reproduce NESA wording; italicised lesson WM indicators are teacher-designed applications.

All offer terms are fictional classroom data, not current financial-product or legal information. Each lesson totals 60 minutes. Support should respond to observed student work, and all learners retain the core calculation and reasoning goals.