Mental Math Tricks: Master 29 Shortcuts and Fix 2 Faulty Flashcards

Mental math is not a talent, it is a short list of rewrites: turn the awkward number into a friendly one (10, 50, 100), compute, then correct. The Flip It deck Mental Math Shortcuts & Tricks (deck code ZE6925, 47 cards) packs those rewrites into its first 29 cards, then drifts into trivia. Here is how to get the most out of it.

What the deck actually contains

• Cards 1-29, the toolkit: multiplying by 5, 9, 10, 11, 12, 25, 50 and 99, squaring numbers ending in 5 or 1, squares near 50, numbers near 100, the difference of squares, left-to-right addition, compensation, doubling and halving, Russian peasant multiplication, quick percentages (5%, 10%, 15%, 20%) and divisibility tests for 3, 4, 6, 8 and 9.
• Cards 30-39, the culture block: subitizing, the parietal lobes, the phonological loop, the Mental Calculation World Cup, 13th-root extraction, the mental abacus (soroban), the Doomsday algorithm, lightning calculators.
• Cards 40-47, facts to store: decimal-to-fraction pairs, squares up to 20², cubes ending in 5, square-root estimation, hypercalculia, plus a Rain Man question.

Eight cards worth drilling first

Two cards to rewrite before you study

Card 44 (cubes ending in 5) is wrong. Its worked example gives 1×2×125 + 1×125 = 375, yet 15³ = 3375, and the claim that such cubes end in 125 or 875 fails on 15³ and 25³ = 15625. Replace it with a plain fact: 15³ = 3375, 25³ = 15625, 35³ = 42875.

Card 27 (Russian peasant) stops halfway. Write the full table: halve 23 (23, 11, 5, 2, 1) and double 17 (17, 34, 68, 136, 272), cross out rows where the halved number is even, and add the rest: 17 + 34 + 68 + 272 = 391, the same answer card 17 gets with a² − b². Two methods, one result: that is a free self-check.

Two smaller fixes: on card 26, halve the even number instead (16 × 35 = 8 × 70 = 560), and on card 8, the order is ÷4 then ×100.

Study tips that fit this deck

1. Turn every trick into a fresh problem. The backs show one example each, so a card you "know" may only mean you memorized 47 × 9. Before flipping, invent your own number (63 × 9? 86 × 11?) and solve it aloud.
2. Treat cards 30-39 and 46-47 as a separate, light deck. They are recognition facts, not skills; review them weekly and give your daily reps to cards 1-29 and the squares on card 43. The Doomsday card (38) names the method without its anchor dates, so add one card per anchor (4/4, 6/6, 8/8, 10/10, 12/12) if you want it to work.

Keep going

Squares, cubes and roots are the raw material for half of these shortcuts: pair this deck with How to Memorize Mathematical Constants, Squares, Cubes and Roots, and with Master the Multiplication Tables 1 to 12 if the basic facts still slow you down.

Search for ZE6925 in the library on Flip It to start.