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40 questions · full solutions

Mental Math

Technique, not grinding: difference of squares, fraction wheels, percentage flips, and the anchors that make timed arithmetic screens feel slow.

  1. 0147 × 53 in Two SecondsEasy
  2. 02Squaring Numbers Ending in 5Easy
  3. 0318% of 450, No PaperEasy
  4. 04The Sevenths WheelMedium · Private
  5. 05Rule of 72Easy
  6. 06The 11-Times SlideEasy
  7. 07Double One, Halve the OtherEasy
  8. 0897 × 96 Without GrindingEasy
  9. 09Squares Near 50Medium · Private
  10. 10Up 20%, Down 20%Easy
  11. 11The Thirteenths WheelMedium · Private
  12. 12Divide by 5, 25, 125 at SightEasy
  13. 13√60 by InterpolationMedium · Private
  14. 14How Many Seconds in a Year?Medium · Private
  15. 15The Sixteenths LadderEasy
  16. 164824 × 25 Before the Cursor BlinksEasy
  17. 17Basis Points to Dollars, InstantlyEasy
  18. 18996² and the Thousand AnchorMedium · Private
  19. 19× 99: Shift, Then SubtractMedium · Private
  20. 20530 ÷ 7 to Three Decimals, No Long DivisionMedium · Private
  21. 21Down 36%: What Gain Gets You Back?Medium · Private
  22. 22Rule of 114: Tripling TimeMedium · Private
  23. 23VWAP by See-Saw, Not by SumMedium · Private
  24. 245.6% of 12.5, FlippedMedium · Private
  25. 25When to Skip on a +1 / −2 ScreenMedium · Private
  26. 2668 × 77: Pick the Split That Carries LeastHard · Private
  27. 277 × 4,683 Left to RightHard · Private
  28. 28Tick Math: 102-27 in DecimalHard · Private
  29. 2956 × 375: The Disguised EighthHard · Private
  30. 30Three Percent Moves, Zero DecimalsHard · Private
  31. 311.02^10 Without a CalculatorHard · Private
  32. 32√54.8 from a Decimal AnchorHard · Private
  33. 33Cube Root of 103,823 in Five SecondsHard · Private
  34. 34ln(1.06) to Within a Basis PointHard · Private
  35. 35How Many Digits Has 2^50?Hard · Private
  36. 36A Messy Chain: 0.00042 × 3,600,000 ÷ 0.012Hard · Private
  37. 37Fair Value from a Lopsided BookHard · Private
  38. 38Stripping the Vig from −140 / +120Hard · Private
  39. 39The Two-Modulus Error TrapHard · Private
  40. 40EURJPY from Two Legs, No Long MultiplicationHard · Private