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 ...

Methods similar to this go back thousands of years, at least to the ancient Sumerians and Egyptians. But is this really the best way to multiply two big numbers together? Around 1956, the famous ...