MENTALMATH Logs

Add the same amount to both numbers to make the one you're subtracting a round number — the difference never changes. Because (a + k) − (b + k) = a − b, shifting both ends by the same k leaves the gap identical: 4.32 − 1.95 → add 0.05 to both → 4.37 − 2.00 = 2.37. Check: 4.32 − 1...

Quiz: MENTALMATH / Addition & Subtraction | updated: Jul 30, 2026

864 — add 300, then subtract 4. 568 + 296 → 568 + 300 = 868 → 868 − 4 = 864. Because 296 is just below 300, compensation is less work than adding 200 + 90 + 6 separately.

Quiz: MENTALMATH / Addition & Subtraction | updated: Jul 30, 2026

3,211 — use complements: 9,999 − 6,789 = 3,210, then add 1. A fast route for subtracting from a power of 10: 9,999 − 6,789 = 3,210 because each digit complements to 9. Since the real top number is 10,000, add 1: 3,210 + 1 = 3,211. This is just counting up in disguise: 6,789 → 9,9...

Quiz: MENTALMATH / Addition & Subtraction | updated: Jul 30, 2026

No — it's a 20th-century system compiled by Bharati Krishna Tirtha (1884–1960) and first published posthumously in 1965; it has essentially nothing to do with the actual mathematics of the Vedic period. Tirtha said he derived 16 "sutras" from the Vedas between 1911–1918, but scho...

Quiz: MENTALMATH / Checking & Vedic | updated: Jul 30, 2026

A number is divisible by 3 if its digit sum is — because every power of ten leaves remainder 1 when divided by 3. Since 10 ≡ 1 (mod 3), each digit contributes just itself to the remainder, so the whole number ≡ its digit sum (mod 3): 405 → 4 + 0 + 5 = 9 → divisible by 3. ✓ 1,238...

Quiz: MENTALMATH / Divisibility Rules | updated: Jul 30, 2026

Split the two digits apart and drop their sum in the middle — carrying if that sum reaches 10. Because ×11 = ×10 + ×1, the middle digit becomes the sum of the outer two: 25 × 11 → 2 (2+5) 5 → 275. 72 × 11 → 7 (9) 2 → 792. The carry pitfall (the most mis-stated trick online): if...

Quiz: MENTALMATH / Multiplication Tricks | updated: Jul 30, 2026

Use surpluses instead of shortfalls (and add); if the right part exceeds two digits, carry the extra into the left part. Above 100: 103 × 104 → surpluses +3, +4 → left 103 + 4 = 107, right 3 × 4 = 12 → 10712. (Check: 10712 ✓.) Right-part carry: 88 × 88 → shortfalls −12, −12 → le...

Quiz: MENTALMATH / Multiplication Tricks | updated: Jul 30, 2026

You can flip which number is the percentage — because a%·b = (a·b)/100 = b%·a — turning a hard percentage into an easy one. 8% of 25 looks awkward → swap to 25% of 8 = a quarter of 8 = 2. 4% of 75 → 75% of 4 = 3. 18% of 50 → 50% of 18 = 9. Both sides are literally the same prod...

Quiz: MENTALMATH / Percentages & Estimation | updated: Jul 30, 2026

Cross-multiply for the numerator, multiply the denominators: a/b + c/d = (ad + bc) / bd. ¾ + ⅖ → (3·5 + 4·2) / (4·5) = (15 + 8) / 20 = 23/20. ⅓ + ¼ → (1·4 + 3·1) / 12 = 7/12. Check: 23/20 = 1.15 = 0.75 + 0.40. ✓ The same pattern with a minus subtracts: a/b − c/d = (ad − bc)/bd....

Quiz: MENTALMATH / Percentages & Estimation | updated: Jul 30, 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 30, 2026