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Flash sum

Anzan-style: a burst of numbers flashes past, fast; give their sum. More numbers, more digits each, less time each — the width never stops.

  • Calculation
  • Timed
  • Endless levels

How it plays

Numbers of the same length flash one after another, each for well under a second. Add as they come and give the sum when they stop. The abacus schools call this anzan; the trick is to keep one sum, not a list.

Levels add a number (up to fifteen), a digit to every number (without limit) or take time off each flash (down to a quarter of a second). Typing the answer is the honest form; choosing lets you check rather than add.

The wrong options are a flash missed, a flash counted twice, and a carry dropped or added in one column. The working adds the columns right to left.

Keys

Space starts and skips ahead · 1–9 pick · Enter submits · Esc stops. On a phone, tap.

What it trains

This trains fast addition of numbers seen once. Practice makes this task faster; it is not a measured gain in general memory or intelligence.

Learn

How to get better at flash sum: the method, the techniques, what each new level asks of you, and the traps the wrong answers are made of.

Your approach to each question

  1. Read the level line before you start: it says how many numbers are coming and how many digits each has, so you know the size of the answer before the first flash.
  2. Hold one number only — the total so far. When a flash appears, fold it into the total and let the flash go; never try to remember the numbers themselves.
  3. Add each flash as soon as you have read it, and say the new total to yourself once, silently. Saying the old total again wastes the time before the next flash.
  4. When the flashes stop, the total is your answer. Type it straight away: the answer time is short, 5 seconds plus about a second per digit.

Techniques and tricks

  • Add from the left

    Add the biggest place first, then the next. You keep a usable total even if the last digits are rushed.

    e.g. Total 47, flash 38: 47 + 30 = 77, then + 8 = 85.

  • Make tens

    With one-digit flashes, watch for pairs that make ten (7 and 3, 6 and 4) and for totals that land on a ten; round totals are easier to hold.

    e.g. 6, 4, 9: 6 + 4 = 10, then 19.

  • Round and compensate

    Add a near-round number as the round one, then correct.

    e.g. Total 126, flash 98: 126 + 100 = 226, − 2 = 224.

  • Groups of three digits

    Long numbers flash with commas. Add each three-digit group into the matching group of your total, carrying between groups, rather than reading a long string of digits.

    e.g. 12,480 + 7,615: 12 + 7 = 19 thousand, 480 + 615 = 1,095, so 20,095.

  • Picture the beads

    Anzan is adding on an imagined soroban, the Japanese abacus: you move beads in a picture instead of saying numbers. It is the method the abacus schools teach, and it takes long practice on a real abacus first.

  • Size check

    The first digit of each number averages about 5, so n numbers of d digits add to roughly n × 5.5 × 10^(d − 1). Use it to catch a total that is a whole flash too big or small.

    e.g. 6 two-digit numbers come to about 6 × 55 = 330.

As the levels climb

  • Three things take turns: one more number (from 3 up to 15), one more digit on every number (no limit), and a shorter flash, from 1,000 ms down to 250 ms by level 24.
  • When the flash time drops, the gap after each flash stays 110 ms, so most of your adding moves into the flash itself. Start adding as soon as you have read the leading digits.
  • When the digits grow past three, switch to adding in three-digit groups, and accept that the last group is where errors creep in; check it against the size estimate.
  • From 15 numbers on, only the digits grow, by one a notch. The work is wider sums at the same speed, so practise the group method on its own until it is automatic.

Traps to avoid

  • A flash missed. The total comes out short by exactly one of the numbers shown. It usually happens while you are still adding the one before; read the next flash first, then finish.
  • A flash counted twice: the total is over by one of the numbers. It happens when you lose your place and add the same flash again. Add each flash once, as it appears, and move on.
  • A carry dropped or added in one column: the total is out by 10, 100 or another power of ten. The last digits are right, so check the leading digits against the size estimate.

How to practise

  • Keep Answer by on “Typing the number”. Choosing lets you check options against a vague total rather than add, which trains a different skill.
  • If the flashes are too fast to read, set “Digits at level 0” back to 1 and build speed on single digits before width.
  • Raise “Digits at level 0” to 2, 3 or 4 when single digits feel easy, so you practise wider numbers without climbing through the small ones again.
  • When you miss, read the working’s “Shown” line against your total: short by one number is a missed flash, out by a power of ten is a carry. Practise the one that keeps happening.

Levels

Each question has a clock that comes from the work it asks for. There is no last level: at least one dial turns without end. Your level is judged on your last ten answers, every ten: 90% moves you up, below 80% moves you down, which holds most people near four in five right.

All of it is in the settings beside Start — how many answers a judgement looks at and how often it recalibrates, the thresholds, the time, which questions, how many options, and whether you see each verdict at once or review them together at the judgement. You can also jump to any level.

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