Numberphile revived an ancient multiplication trick—halves and doubles—also called Egyptian or Russian math, where you repeatedly halve one number and double the other. After crossing out rows with ...
What are you like at multiplications? Chances are if you're a mere mortal 2, 5s and 10s won't seem too taxing. But when it comes to figuring out what 42x21 is, you may wish somebody had shown you the ...
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