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- multiplying a number by 5 is halving and multiplying by 10
- can get to any number this way - eg, multiplying by 7 is 5 times plus double
- multiplying by 11, or 22 or 33 & the 11 divisibility rule
- squaring 2-digit numbers that end in 5, or same first digit & second digits sum to 10 (eg, 22 x 28)
- switching percentages
- difference of squares
- summing 1 to x using Gauss or Euclid’s method
- binomial expansion
- squaring a 2-digit number - 45^2 = 40^2+40*5*2+5^2
aug 23 2026 ∞
aug 31 2026 +
aug 31 2026 +