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 ...

An animated guide in which fractions multiply to make baby fractions and a rather violent fraction divides numbers with a chainsaw. Use to illustrate the processes of multiplying fractions, and ...

19 is only 1 away from 20, so you prefer to start by multiplying 20 times 5, which equals 100. Then we need to take away a squadron of 5, because there are actually only 19 squadrons, not 20. 100 take ...