Charlie Chang’s apps

Dead-Time Learning Engine

Use unused time for learning and retain what was learned.

Uses the gaps - the commute, the queue, the ten minutes before a meeting - to teach something, and then makes sure it sticks. It picks material sized to the gap it has found, and brings things back on a spacing schedule rather than assuming one pass is enough.

What a v1 does5 features
  • Sessions sized to the time actually available, from two minutes up
  • Spaced repetition, so what was learned is returned to
  • Audio-first mode for hands-busy time
  • Material drawn from what the user is already trying to learn
  • A weekly report of what was retained, not what was consumed

The market

Proven marketNotes to study material
$139Ma year at Quizlet, from 50M monthly users

Goodnotes reports 30M monthly users and Notability earns an estimated $3M a month. Quizlet Plus costs $7.99, so $7 a month sits inside an established price band.

Where these numbers come from
CompanyEarnsReachSource
Quizlet$139M revenue, 2025 · Plus is $7.99/mo50M+ monthly active usersReported, 2025
GoodnotesNot published30M+ monthly active usersEstimates span $40M to $100M+, too wide to state
Notability~$3M a month~400k downloads a monthSensor Tower estimate, Mar 2026

Why this audience

This audience treats commutes and gaps as wasted inventory, which is a specific mindset. They are already listening to something in those minutes. Turning that time into retained learning rather than consumed content is a distinction they will understand instantly.

Before it can launch

No licence, certification or data agreement stands between this and a launch — app-store review and an ordinary privacy policy aside.

What it earns

Drag me Every figure recalculates as you go.

Paying users1,000

0.05% of 2M followers · capped at 50,000

Price per month$7

Opens on the price this idea was costed at, $7.

1,000 paying — 0.05% of 2M followers — × $7 a month = $7,000 a month

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Revenue a month$0123456789,012345678901234567890123456789
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Fifty thousand paying users is the ceiling every one of these pages is drawn against, not a forecast. It opens at a thousand, the bottom of the range, so the first figures you see are the modest ones. What the cap buys is comparability: the same range on every app, and the percentage of the audience it takes as the number that moves.

Is this the one for your audience?

We fund it, build it, and run it. You bring the audience and take a share of the revenue. Nothing to pay and nothing to build.

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