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Math, from the beginning

Mental Math Essentials

Times tables, tips, and percentages: the tricks school skipped.

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What this covers

Every lesson is short enough to finish in one sitting and ends with questions that make you use what you just read.

  1. You Were Never Bad at MathHow this works, and a warmup4 min
  2. Adding & Subtracting in Your HeadLeft to right, friendly tens, round and adjust5 min
  3. The Easy Half of the Table×1, ×2, ×5, ×10, and the halving secret5 min
  4. Doubling: ×4 and ×8Double once, twice, three times4 min
  5. Building: ×3 and ×6Double plus one more, then double that4 min
Show all 15 lessonsShow fewer lessons
  1. The Nines Trick and Stubborn Sevens×9 and ×7 without tears5 min
  2. Elevens, Twelves, and SquaresThe last corner of the table4 min
  3. Division Is Multiplication BackwardsSame table, read the other way4 min
  4. Fractions and Percents Are the Same ThingThe family tree5 min
  5. The 10% and 1% Building BlocksEvery percent is built from two moves5 min
  6. Any Percent of Any NumberStack the blocks, or flip the problem5 min
  7. Tips Without a CalculatorThe restaurant superpower6 min
  8. Discounts and Sales TaxWhat you actually pay5 min
  9. Percent Change, Both DirectionsRaises, drops, and 'what percent is that?'6 min
  10. Mental Math Timed Simulation: Set A30 items, 15 minutes, exam mode15 min

A whole lesson, start to finish

Not a sample and not a summary. This is all of lesson 2, exactly as it reads in the app.

Lesson 2 of 15 · about 5 min

Adding & Subtracting in Your Head

Left to right, friendly tens, round and adjust

Strategy 1: left to right

63 + 28: tens first (60 + 20 = 80), then ones (3 + 8 = 11), so 91. Say the running total in your head, not the digits.

Strategy 2: make a friendly ten

Numbers ending in 8 or 9 are begging to be rounded. 49 + 27: treat 49 as 50, get 77, take back the 1: 76. You borrowed, so you repay.

Strategy 3: subtract by adding up

83 - 57: instead of borrowing, climb from 57. Up 3 to 60, up 23 more to 83: total climb 26. Subtraction is just measuring the gap, and climbing is easier than cutting.

Real life

This is the checkout-line skill: is $63 + $28 more than the $90 in your budget? Tens first gives you 80, the ones add 11, so 91: just over. Two seconds, and the close calls are exactly when you finish the ones.

Then you drill it

This topic does not use multiple choice. Every lesson ends with problems you work out and type the answer to, freshly generated each time, because real life does not offer four options. Miss one and it shows you the trick again and brings that problem back sooner.

Terms you will own

Cards come back on a schedule built from how well you knew them last time, so the ones you keep missing show up more.

More in math, from the beginning

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