MathDrill

n²: tens digit T → T×(T+1), append 25
35² → 3×4=12 → 1225 · 85² → 8×9=72 → 7225

d² = (d−x)(d+x) + x² (x = distance to nearest 10)
43² → x=3 → 40×46+9 = 1840+9 = 1849

AB×11 → A_(A+B)_B (carry if A+B≥10)
45×11 → 4_9_5 = 495 · 78×11 → 8_5_8 = 858

Halve one, double the other until round.
35×12 → 70×6 → 140×3 = 420

16×25 = 4×(4×25) = 4×100 = 400
8×125 = 1000 · 4×125 = 500 — memorise!

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

Hourly × 2000 ≈ annual (40hr/wk, 50wks)
$30/hr × 2000 = $60k/yr

10%: move decimal · 15%: 10%+5% · 20%: 10%×2
$84 → 10%=$8.40 → 15%=$12.60

Round subtrahend up, compensate after.
84−39 → 84−40+1 = 45

Work biggest place first.
57+36: 57+30=87, +6 = 93