Siberian/Russian Peasant MultiplicatioN
JHD was taught this, in 1958 as far as he can remember, by Besicovitch
There is a good general description in WikiHow. However, this fails to mention a key advantage which Besicovitch taught JHD in the context of the UK's pre-decimal currency: it works well when the multiplicand is expressed in multiple units. These days this is most clearly seen when the multiplicand is time.
Example with time
One egg boils in 3 minutes 10 seconds. Assuming you can only boil one egg at a time, and the change is instantaneous, how long will it take to boil 11 eggs?
| Number | Time |
| (Halving) | (Doubling) |
| 11 | 03m10s |
| 5 | 06m20s |
| 2 | 12m40s |
| 1 | 25m20s |
We cross out the even rows (in fact there's only one), and add up the remaining times:
| Number | Time |
| (Halving) | (Doubling) |
| 11 | 03m10s |
| 5 | 06m20s |
| 1 | 25m20s |
| | ====== |
| Total | 34m50s |