Mental math guides
Short, practical guides to calculating faster in your head. Each one pairs with a timed drill in R3CK0N so you can practise what you've just read.
Flashcards
Spaced-repetition study, powered by the same FSRS scheduler as modern Anki. Build your own decks or import existing Anki decks directly.
Spaced repetition, explained
Why flashcards beat cramming, what a review actually is, and how the Again/Hard/Good/Easy buttons decide when you'll see a card again.
Study your Anki decks in the browser
Import an Anki .apkg or Notes in Plain Text export: what carries over, what doesn't, and how the statistics compare.
FSRS, the scheduling algorithm behind modern Anki
Stability, difficulty, retrievability and desired retention — how FSRS decides when a card comes back, without the math.
How to write flashcards that actually stick
The minimum information principle, cloze deletions, and before-and-after examples of cards rewritten to stick.
Flashcards for mental math
Why memorized facts speed up calculation, plus the built-in starter decks: squares, cubes, powers of two, times tables and more.
Cloze deletion flashcards
What a cloze card is, when a blank beats a front-and-back card, and how to write deletions that test recall rather than grammar.
Desired retention, explained
What the desired retention setting really controls, the trade-off between how much you remember and how much you review.
How many new cards a day?
Every new card is a promise of future reviews. The review-load math behind the daily limit, and what 5 to 40 a day costs you.
Reviewing every day
Habits, streaks and reminders for actually doing your reviews, plus the 4am cutoff and why one missed day is not a disaster.
Speed tests
Addition · Subtraction · Multiplication · Division · Numpad typing
Printable math worksheets
Make a fresh addition, subtraction, multiplication, division or mixed worksheet at four levels, with an answer key.
Quant interview prep
Timed arithmetic tests, probability and expected value, sequences, estimation and market making: what gets tested and how to practise.
GRE prep: Quant mental math and vocab flashcards
The Quant facts to know cold, mental math that beats the on-screen calculator, and free GRE Quant and vocabulary decks.
How to get faster at mental math
Seven steps that work: automatic facts, a few techniques, and short timed practice aimed at your slowest facts.
Mental math as brain training
A free ten-minute daily routine to keep your mind sharp: a sprint, your slowest facts, the daily challenge, one technique a week.
Mental math for students
What to practise at each stage of school, a ten-minute plan for school days, and how a class can use it.
A Zetamac alternative
The same two-minute mixed sprint, plus per-problem timing, history and targeted drills.
Free mental math websites, compared
Zetamac, DrillYourSkill, TraderMath and R3CK0N side by side: price, timing, what each covers, and who it suits.
80 in 8 practice test
Eighty mixed arithmetic questions against an eight-minute clock, playable on the page.
Mental math for trading interviews
How to prepare for the timed arithmetic tests used in quant and trading interviews.
Mental math practice
How to practise mental arithmetic so it actually improves, plus timed drills for all four operations.
How to multiply two-digit numbers in your head
Three methods — splitting, rounding and the difference of squares — with worked examples.
Mental math tricks
Multiply by 11, 9, 5 and 25 instantly, square numbers ending in 5, and estimate square roots.
Times tables practice
A timed multiplication test up to 12 × 12, and the order to learn the tables in.
Square roots from 1 to 150
Every square root to four decimal places with its simplest radical form, and a page for each showing how to estimate it in your head.
Times tables 2 to 20
A page for every table from 2 to 20: the answers, the trick that makes it easy, and a timed twelve-question quiz.
Multiplication chart 1–12
Every times table on one printable grid, plus a chart to 20 × 20 and the patterns that make it easy to learn.
How to square numbers in your head
One method for every two-digit square, plus shortcuts near 50 and 100.
Square numbers from 1 to 100
Every square from 1² to 100² in one table, the perfect squares up to 1,000, and the patterns behind them.
How to estimate square roots in your head
Bracket it between perfect squares, then one divide-and-average step gets within 0.04.
How to calculate percentages in your head
Tips, discounts and percentage change from three building blocks: 10%, 5% and 1%.
Prime numbers from 1 to 100
All 25 primes in a chart, a four-check test for any number up to 120, and the look-alikes (51, 57, 87, 91).
Powers of 2, from 2⁰ to 2³²
The full table, and how 2¹⁰ ≈ 1,000 lets you rebuild any power in your head.
Cube numbers from 1 to 20
1³ to 20³, and the last-digit trick for cube roots of perfect cubes.
How to multiply by 11 in your head
Split the digits, put their sum in the middle, and carry when it hits 10.
Estimation tricks for mental math
Round in opposite directions, count the zeros, and the rule of 72.
Mental math games
Every game in the R3CK0N Arcade, plus the daily challenge and ranked duels.
Fraction to decimal chart
Every fraction up to twelfths, repeating decimals marked, and the patterns for sevenths, ninths and elevenths.
Divisibility rules from 2 to 12
Tell whether a number divides evenly without dividing — including the rules for 7 and 11.
The day of the week trick
Name the weekday of any date from 1800 to 2199 with four additions and a division by 7.
How to memorize numbers with the major system
Turn digits into sounds, sounds into words, and words into pictures — with the full code and a starter list.
Numpad typing test
A free 10-key speed test, measured in keys per second, with tips for typing numbers faster.