Percentage Discount Speed Drill
Crush the "% off" tag in your head. 60-second sprints of how much do you save? and what's the sale price? across the 10–70% pool — plus the stacked-discount trap on Hard. No calculator allowed.
How it works
- Pick your pool. Tap the chips under Discount rates in the pool to choose which percentages you want to drill. By default it's 10 / 20 / 25 / 50% — the four cleanest mental shortcuts. Add 15, 30, 40 for grocery and apparel; add 60 / 70 for clearance racks. Keep the pool small at first; you'll get more reps on each rate.
- Pick a difficulty. Easy rolls whole-dollar prices ($10–$80), mostly asks how much do you save?, and accepts answers within ±$0.50 — perfect for warmup or for kids learning the 10%-rule. Medium uses retail-style prices ending in .99 / .95 / .49 (e.g. $47.99) with a ±$0.25 band, and mixes savings and sale price prompts 50/50. Hard tightens to ±$0.10 and adds a 1-in-5 stacked discount prompt — "$120, 30% off, then extra 20% off at checkout." All three modes draw the discount rate from your configured pool: the difficulty controls price shape, prompt mix, and accept-window, not which percentages get tested.
- Start the sprint. Tap the big Start Sprint button. The timer counts down (60 seconds by default; configurable). A price and a discount appear; type the answer and press Enter (or tap Check). Stuck? Tap Skip → (or press Esc) to flash the answer and move on. Skip resets your current streak but doesn't drag down your accuracy — only answers you actually submit count toward the round-end accuracy %.
- Tightest answers earn more. A correct answer is worth a base 10 points. Bonus: +5 if you answered in under 5 seconds, +3 under 10, +1 under 20. A correct-streak of 3+ adds the streak count as bonus on each subsequent correct answer. The accept-window is ±$0.50 on Easy, ±$0.25 on Medium, ±$0.10 on Hard.
- Wrong? You'll see the shortcut. If you miss, the feedback line shows the correct amount and the mental-math shortcut you should have used ("25% = quarter of $48 → $12 saved → $48 − $12 = $36"). Stacked-discount misses show the sequential breakdown so the trap clicks. You don't lose points for mistakes, but the streak resets.
- Round-end summary. When the timer hits zero, a modal lands with your score, accuracy, best-streak-this-round, and your personal best. Tap Play again to roll another sprint immediately.
What this trains — and why "% off" math matters
The clearance rack says 30% off, take an extra 20% at checkout. The receipt shows a number. Whether you got a good deal — whether the deal you took at the next store would have beaten this one — depends on being able to do the math in the aisle, not at home with a spreadsheet. The point of this drill is to make that math automatic, in about fifteen minutes of practice spread across a couple of weeks.
The mechanic is deliberately tight: random price, fixed discount, type the answer. No menus, no multi-step UI — just the loop that builds the reflex. The two prompt types ("how much do you save?" and "what's the sale price?") cover the two real-world questions a shopper actually asks; the speed bonus + streak multiplier reward the "I already know this" state we're optimizing for. The mental-math cheatsheet is always visible, so by the third or fourth round you'll have internalized the 10%-rule + adjustments (quarter for 25, halve for 50, sequential-not-additive for stacked) without having read the cheat sheet on purpose. There is deliberately no lifetime level ladder — this is a fluency drill, not a points grind. Once you can clear a Medium round with 90%+ accuracy in under 10 seconds per price, you've gotten what the app exists to give; come back occasionally to keep the reflex warm.
Features
- 60-second sprint mechanic — sized like one moment in a store aisle; long enough for a real workout, short enough to fit between aisles.
- Configurable discount-rate pool — toggle 10 / 15 / 20 / 25 / 30 / 40 / 50 / 60 / 70% on or off per chip. The drill draws only from rates you've opted in to.
- Two prompt types mixed by difficulty — how much do you save? (price × rate%) and what's the sale price? (price × (1 − rate%)). Easy weights savings prompts more; Medium and Hard mix them 50/50.
- Stacked-discount Hard-mode wrinkle — 1-in-5 questions on Hard ask "$X, A% off, then extra B% off at checkout — final price?" The classic trap that A% + B% ≠ (A + B)% off. After a wrong answer the feedback shows the sequential breakdown ($100 → 30% off → $70 → 20% off → $56, effective 44%).
- Three difficulty modes with tiered tolerance — Easy (whole-dollar prices $10–$80, ±$0.50, savings-heavy), Medium (prices ending .99/.95/.49, $10–$200, ±$0.25, balanced mix), Hard (same shapes $10–$300 plus stacked discounts, ±$0.10). The discount pool is the same across all three; difficulty controls price shape, prompt mix, and accept-window.
- Score + time bonus + streak multiplier — fastest answers earn the most, and a hot streak adds compounding bonus points so the difference between "kept up" and "crushed it" is visible in the score.
- Per-round and all-time stats — score, accuracy, best-streak-this-round in the round-end summary; best score ever and best-ever streak persist in the hero card across rounds.
- Mental-math cheatsheet — always-visible reference card with six shortcuts: 10% rule, double-for-20, quarter-for-25, halve-for-50, savings-then-subtract for sale price, sequential-not-additive for stacked. Reads like a flashcard set you can't lose.
- Skip when stuck — tap Skip → (or press Esc) to flash the answer and the shortcut, then advance immediately. Skipping resets your current streak but doesn't drag down your accuracy.
- Currency switcher — display-only currency symbol (USD, EUR, GBP, CNY / RMB, JPY, CAD, AUD, INR, SEK / NOK / DKK, BRL) so the prices read right wherever you shop.
- Light and dark themes — auto-follows your OS preference, with a manual toggle in the header.
- Local-only storage — your score history lives in this browser only; nothing is uploaded.
- One-page design — single static HTML page, no install, no app store, no signup, no email field.
Tips for productive practice
Pick a small pool and stick with it. The fastest way to get good is to drill one discount rate until it's automatic, then add the next. The cleanest ones are 10 / 25 / 50 — if you can do those without thinking, most clearance racks become legible by inspection (35% off ≈ 25% off + a bit; 60% off = halve, then a bit more). Add 15 / 30 next; save 40 / 60 / 70 for once the base ones are reflexive.
Don't chase speed at the cost of the shortcut. The score formula rewards fast correct answers; a string of fast-wrong answers nukes your streak and your round score. Slow down to think the shortcut out loud the first few times (in your head, mouthing the steps), then speed up as the path becomes automatic. The point is the reflex, not the wall-clock time.
Spend extra time on the stacked-discount prompts. The "30% then extra 20% ≠ 50% off" trap is the single biggest in-store math error people make — and the single most valuable thing this drill can teach. When a stacked prompt fires on Hard, take the time to walk through the sequential breakdown in your head before answering. After half a dozen reps the shortcut "stacked discounts compose, they don't add" will be there forever.
Use round breaks as a sanity check. When the timer hits zero, look at accuracy — that's the number that matters. Score gets inflated by speed bonuses; accuracy is the truth-teller. Below 80% means you're guessing; drop a difficulty or trim the pool until accuracy comes back into the 90s, then re-expand.
Frequently asked questions
Do I need to create an account?
No. There is no sign-in, no email field, and no profile. Everything you enter is saved only in this browser's local storage on your own device. Nothing is sent to any server.
Why does the drill mix two prompt types?
Because those are the two real-world questions a shopper actually asks. How much do you save? is one multiply (price × rate%) and is the cleaner mental drill — it's how Easy mode warms you up. What's the sale price? is the question you actually need at the register, and it's a multiply-then-subtract — slightly harder but more useful. Medium and Hard mix both 50/50 so you stay fluent at the harder one without losing the easier one.
What's the deal with stacked discounts?
Stacked discounts (sometimes called compound discounts) are when a store advertises a base discount and then takes an additional percentage off the already-discounted price at checkout. The crucial point — and the single biggest math error consumers make — is that the percentages don't add. "30% off, then extra 20% off" is NOT 50% off. It's 30% off the sticker, then 20% off whatever's left. On a $100 item that's $100 → $70 → $56. Effective discount: 44%, not 50%. Hard mode fires a stacked prompt 1-in-5 to drill exactly this reflex. The mental shortcut for two reasonably-sized stacked discounts is "the bigger one, plus about a quarter of the smaller" — it's not a perfect rule but it gets you within a few percent of the truth in your head.
How does the score formula work?
Each correct answer earns a base of 10 points, plus a speed bonus: +5 if you answered in under 5 seconds, +3 under 10, +1 under 20. Once your correct-streak hits 3 or more, you also get the current streak number added as a multiplier bonus on each subsequent correct answer. A wrong answer doesn't subtract points but does reset the streak to zero. Each round's score is independent; the hero card tracks your best ever and your best streak ever across rounds.
Why is there no lifetime level or rank system?
Because this is a skill drill, not a points grind. The thing you're training — fluent mental discount math — has a real ceiling: once you can clear a Medium round at 90%+ accuracy with sub-10-second answers, the reflex is built. Adding a lifetime rank ladder would push you to keep grinding for points after the skill was already locked in, which is the opposite of what this app exists for. The round-end summary celebrates the round; the hero card tracks personal bests; that's the whole reward loop.
How is "correct" defined? Why such a wide accept-window on Easy?
The accept-window is tiered by difficulty to match the mental shortcut a real shopper at that level would actually use:
- Easy: ±$0.50 — captures rounding to the nearest dollar. On a $30 × 20% prompt, anything from $5.50 to $6.50 counts. This is what most people do in the aisle: read the 10% off the cents column, double it, round up.
- Medium: ±$0.25 — captures rounding to the nearest quarter or dime. On $47.99 × 30% = $14.40, anything from $14.15 to $14.65 counts.
- Hard: ±$0.10 — cents-column precision. On the same $14.40 target, only $14.30–$14.50 counts. This is for people who want the math drill in its strict form.
The wider Easy band isn't a freebie — it reflects that on whole-dollar prices, the natural mental shortcut rounds to friendly numbers. All three modes draw the discount rate from the same pool you configured; difficulty only changes price shape, prompt mix, and how strict the accept-window gets.
Does difficulty change which discount rates I'm tested on?
No. Difficulty and the discount pool are independent dimensions. The pool decides which percentages appear; difficulty decides how messy the price is, which prompt types fire, and how tight the accept-window is. If you keep 70% in your pool on Easy, you'll still get "$30 × 70% = $21 saved" — just with a generous ±$0.50 band.
What happens if I clear my browser data?
The tracker resets, because progress is stored locally rather than in an account. Switching browsers, switching devices, or using private browsing has the same effect. Screenshot the hero card if you want a permanent record of your best score and streak.
Does it work offline?
Once the page has finished loading, all the drill mechanics work without an internet connection. Reconnecting is only needed to load the page initially or to visit other pages on OmniAppHub.