Day 3: Prototype and numbers
Build something people can touch, then check that the money works.
Today's goal
By the end of today you can build and test a simple prototype and calculate price, costs, margin and break-even.
Agenda
- Warm-upThirty-second sketches10 min
- LearnPrototypes help you think10 min
- ActivityBuild two rough prototypes40 min
- ActivityTest with users25 min
- BreakBreak15 min
- LearnPrice, costs, margin and break-even15 min
- Case studyWorked example: bookmark packs at a school fair20 min
- ChallengeYour numbers on one page30 min
- ReflectDid the numbers change your mind?10 min
- QuizDay 3 quiz5 min
The session
Thirty-second sketches
Quick drawing loosens up perfectionism before you build.
- Set a 30-second timer and sketch your idea as a picture, not words.
- Repeat four times, each time from a different angle: the user's hands, a screen, the moment of use, the result.
- Choose the sketch that best shows the experience.
Solo
Sketch alone, then show the four sketches to someone and ask which is clearest.
Group
Each team member sketches, then the team picks one together in 2 minutes.
Prototypes help you think
Prototypes are quick, cheap and meant to be changed.
- The d.school says: 'We build to think and learn.' A prototype is a question you can hold, not a finished product.
- Three kinds: physical (card, paper, packaging), digital (screens drawn on paper or a free slide app) and experience (acting out the service).
- Show, don't tell: let people use it instead of explaining it.
- The d.school's testing line: 'Prototype as if you know you're right, but test as if you know you're wrong.'
- Comparing two versions usually produces more useful feedback than asking whether someone likes one.
Related lessons: Testing an idea cheaply: customer conversations and simple experiments
Build two rough prototypes
Make two different versions of your idea, quickly and cheaply.
- Choose your format: paper screens for an app, a card mock-up for a product, or a role-play script for a service.
- Build version A in 15 minutes.
- Build version B in 15 minutes, changing one important thing (layout, size, how it is delivered, what comes first).
- Label each with what you want to learn, for example 'Can users find the revision timer within 10 seconds?'
- Photograph both.
- Produce: two prototypes and two learning questions.
Solo
Use paper, card, tape and items from your recycling. Keep everything safe: no hot glue guns or blades without an adult present.
Group
Split the team: two people build A, two build B. Swap for the last 5 minutes to improve each other's version.
Test with users
Watch real people use your prototypes, following the d.school's testing tips.
- Give a tester a realistic task, for example 'You have 10 minutes on the bus and a chemistry test tomorrow.'
- Let them use version A, then version B. Do not explain your thinking or correct what they do.
- Note where they hesitate, smile, get confused or ask a question.
- Ask: 'Which did you prefer, and why?' and 'What would you change?'
- Test with at least three people.
- Produce: a feedback grid with four boxes: what worked, what to change, questions raised, new ideas.
Solo
Test with three family members or friends, one at a time.
Group
Teams pair up and test each other's prototypes, then swap. Testers give honest, kind feedback.
Related lessons: Feedback: giving and receiving it well
Break
Rest before the numbers.
- Rest for 15 minutes.
- Find a calculator and a clean page.
Price, costs, margin and break-even
A great product can still lose money on every sale. These formulas come from the US Small Business Administration (SBA) and work anywhere.
- Variable costs rise with each unit you sell: materials, packaging, printing, delivery.
- Fixed costs are incurred over a period and do not change with how many you sell: a stall fee, an app subscription.
- The SBA defines contribution margin as 'the difference between the price of a product and what it costs to make that product'. As a percentage: (price minus variable cost) divided by price.
- The SBA break-even formula: fixed costs divided by (price minus variable cost) gives the number of units you must sell to cover costs.
- Margin is profit as a share of price; markup is profit as a share of cost. Do not mix them up.
- Count your own time. Lesson ea-04 shows how to price your skills.
Related lessons: The numbers on one page: price, costs, margin and break-even, Pricing your skills and getting paid
Worked example: bookmark packs at a school fair
Work through the numbers step by step. All prices are invented to show the method, not real market rates.
- Omar plans to sell packs of illustrated bookmarks at a school fair. Price: AED 25 per pack. Variable cost per pack: card AED 4, printing AED 4, sleeve AED 2, so AED 10.
- Contribution per pack = 25 minus 10 = AED 15. Contribution margin = 15 divided by 25 = 60 per cent. Markup on cost = 15 divided by 10 = 150 per cent.
- Fixed costs: stall fee AED 100 and a design app for one month AED 50, so AED 150.
- Break-even = 150 divided by 15 = 10 packs. Check: 10 packs bring in AED 250, minus variable costs of AED 100 and fixed costs of AED 150, leaves zero.
- Question 1: Omar's friend suggests a price of AED 20. What is the new break-even? (Answer: 150 divided by 10 = 15 packs.)
- Question 2: If making each pack takes 15 minutes, how many hours does break-even take, and is the project worth it as a business or as a learning project?
Solo
Work through the questions in your notebook, showing every step.
Group
Teams solve it, then one team explains on the board. The facilitator checks the difference between margin and markup.
Related lessons: The numbers on one page: price, costs, margin and break-even
Your numbers on one page
Apply the formulas to your own idea, with every assumption labelled.
- List every variable cost per unit. Find realistic prices online or ask an adult, and mark each as 'checked' or 'guess'.
- List monthly or one-off fixed costs.
- Set a price based on what your customer conversations suggest people value, then calculate contribution per unit and margin.
- Calculate break-even in units. Then recalculate with a 20 per cent discount.
- Add hours: how long does one unit take you?
- Produce: a one-page table you will use in your Day 5 plan. For a free service or social project, calculate what it would cost to run and who could cover that.
Solo
Ask a parent or guardian to check your sums. Do not buy any materials yet.
Group
Teams swap numbers with another team, who recalculate break-even independently. Differences are discussed, not graded.
Related lessons: The numbers on one page: price, costs, margin and break-even, Pricing your skills and getting paid, Selling things and running a small online business
Did the numbers change your mind?
A pause to connect what users said with what the numbers say.
- Write one change you made to your prototype because of feedback.
- Write whether the break-even number feels achievable and why.
- Complete the journal prompts.
Solo
Write for 10 minutes.
Group
Write for 6 minutes, then share one 'the numbers surprised us' moment per team.
Day 3 quiz
Five questions on prototypes and numbers.
- Answer the questions below.
- Redo any calculation you got wrong.
Block timer
Your journal
Private, saved on this device only. Your answers stay in this browser. They are never sent to us, even when you are signed in.
Day 3 quiz
Score 80% or more to complete Day 3.
1. Using the SBA formula, what is break-even if fixed costs are AED 200, price is AED 30 and variable cost is AED 20?
2. How does the SBA define contribution margin?
3. A pack sells for AED 25 and costs AED 10 to make. What is the contribution margin as a percentage of price?
4. What does the d.school advise when testing a prototype with a user?
5. Which completes the d.school line 'Prototype as if you know you're right, but...'?
Overnight challenge
Show your improved prototype to one new person without explaining it. Write down the first thing they say and the first thing they do.