MathDrill

Ends-in-5²: n = AB5 → (AB)(AB+1) then append 25
e.g. 35² → 3×4=12 → 1225

Near-anchor (d²): d² = (d+x)(d−x) + x²
e.g. 43² → pick x=3 → 40×46 + 9 = 1840+9 = 1849

2-digit AB×11: A _ B, middle = A+B
e.g. 45×11 → 4_(4+5)_5 = 495
If sum ≥ 10, carry into left digit.

Keep halving one factor, doubling the other
until one becomes a round number.
e.g. 35×12 → 70×6 → 140×3 = 420

Break one number into easy factors.
e.g. 16×25 → 4×4×25 → 4×100 = 400
e.g. 18×15 → 18×(30÷2) → 540÷2 = 270

Years to double = 72 ÷ annual return %
8% → 72÷8 = 9 years
6% → 72÷6 = 12 years

Annual ÷ 2000 = hourly (40hr/50wk)
$60k ÷ 2000 = $30/hr
Hourly × 2000 = annual

10%: move decimal left
15%: 10% + half of 10%
20%: 10% × 2
e.g. $84 → 10%=$8.40 → 15%=$12.60

Round the subtrahend to nearest 10,
then compensate.
84−39 → 84−40+1 = 45
73−28 → 73−30+2 = 45

Work from biggest place to smallest.
57+36: 57+30=87, 87+6 = 93
423+258: 400+200=600, 20+50=70,
3+8=11 → 681