MENTALMATH Logs

Last-digits for the 2-family, digit-sum for 3 and 9, alternating sum for 11, and combine for composites. Divisor Test 2, 5, 10 last digit 4 / 8 last two / three digits 3 / 9 digit sum ÷ 3 / ÷ 9 6 ÷2 and ÷3 11 alternating digit sum ÷ 11 7 subtract 2× last digit (r...

Quiz: MENTALMATH / Divisibility Rules | updated: Jul 14, 2026

Anchors for building any percent, swap to flip an awkward one, fractions for the "nice" values, and estimation when exact isn't needed. Problem Tool 15%, 35%, tips, discounts 10% / 5% / 1% building blocks Ugly percent of a round number (8% of 25) Swap: x% of y = y% of x...

Quiz: MENTALMATH / Percentages & Estimation | updated: Jul 14, 2026

Match by the number's neighbourhood; fall back to the cross method for anything else. The number… Fastest method Ends in 5 n(n+1) then "25" Near 50 25 ± d, then d² Near 100 (or 1000) number ± d, then d² Near a multiple of 10 (a ± b)² identity None of the above Cros...

Quiz: MENTALMATH / Squaring Shortcuts | updated: Jul 14, 2026

Memorise the halves, quarters, eighths, thirds, fifths — they turn percentage problems into easy fraction ones. Fraction % Fraction % 1/2 50 1/8 12.5 1/4 25 3/8 37.5 3/4 75 5/8 62.5 1/5 20 1/3 33.3̅ 1/10 10 2/3 66.6̅ So "37.5% of 240" is just 3/8 of 240 =...

Quiz: MENTALMATH / Percentages & Estimation | updated: Jul 14, 2026

Pick by the shape of the numbers — a special factor, or closeness to a base. The numbers… Use One factor is 11 Insert-the-sum (with carry) One factor is 5/25/50/125 ×(power of ten) then divide One factor is 9/99/999 ×(round) then subtract One factor halves to somethi...

Quiz: MENTALMATH / Multiplication Tricks | updated: Jul 14, 2026

Each catches a different family of errors; together they're a fast, powerful sanity check, but neither proves correctness. Casting out 9s Casting out 11s Uses digit sum (mod 9) alternating sum (mod 11) Misses transpositions, ±multiples of 9 ±multiples of 11 Catches tr...

Quiz: MENTALMATH / Checking & Vedic | updated: Jul 14, 2026

They're short slogans naming shortcuts you've already used in this module. Sutra Literal meaning The trick Ekadhikena Purvena "By one more than the one before" squaring numbers ending in 5 (n·(n+1) Nikhilam Navatashcaramam Dashatah "All from 9 and the last from 10" multi...

Quiz: MENTALMATH / Checking & Vedic | updated: Jul 14, 2026

Match the trick to the numbers: near a round value → round-and-adjust; two close values → count up; anything → left-to-right. Situation Best tool A number just below/above a round one (398, 497) Rounding-and-adjusting Subtracting two close numbers Counting-up Subtrahen...

Quiz: MENTALMATH / Addition & Subtraction | updated: Jul 14, 2026

836 — round 47 to 50, then subtract the 3 you over-added. One clean route: 789 + 47 → 789 + 50 = 839 → 839 − 3 = 836. This is compensation: make the addend friendly, then undo the small overshoot.

Quiz: MENTALMATH / Addition & Subtraction | updated: Jul 05, 2026

804 — add hundreds, tens, then units into one running total. 426 + 378: Hundreds: 400 + 300 = 700 Tens: 20 + 70 = 90, running total 790 Units: 6 + 8 = 14, running total 804 You still handle the "carry" from 14, but it is absorbed into the running total instead of held as a sepa...

Quiz: MENTALMATH / Addition & Subtraction | updated: Jul 05, 2026