The Ethiopians have used a quite interesting way to multiplicate two big numbers.
First of all they wrote the two numbers (which should be multiplicated) side by side, e.g.:
5468 * 72 .
So this is still the same as we do it today. But the following is quite different. For every following line the left number is divided by 2 (digits behind the comma are omitted), and the right number is doubled for each line. This sequence goes on until the left number is 1.
5468 * 72 ------------- 2734 144 1367 288 683 576 341 1152 170 2304 85 4608 42 9216 21 18432 10 38864 5 73728 2 147456 1 294912
Now all lines with the left number being not an odd number are crossed out.
xxxx---*---xx ------------- xxxx------xxx 1367 288 683 576 341 1152 -xxx-----xxxx 85 4608 --xx-----xxxx 21 18432 --xx----xxxxx 5 73728 ---x---xxxxxx 1 294912
The remaining right numbers are added.
288 + 576 + 1152 + 4608 + 18432 + 73728 + 294912 -------- = 393696
And the sum 393696 is - voila! - the result of the multiplication (5468*72).
Thanks to Anja S. who told me of this!