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!