Recently I wrote and posted a Russian emulator account on the AppStore (there were already several versions of the Roman and Chinese Abacus, but there was no wooden device that was so familiar throughout the entire post-Soviet space). It turned out I think not bad - rolling the bones from the accelerometer, those same sounds and realistic physics, but when I started practicing the counting technique, one big question arose. Addition and subtraction are carried out very simply, and there is nothing to talk about. Multiplication is usually reduced to combinations of addition, subtraction and division. The division is more complicated, there are several interesting techniques (for example, the division into two: moving along the discharges from the bottom up, half of the bones of this category are shifted to the right; if they are an even number, they move to the next, if odd, they are taken away and rounded up five bones) that allow dividing by 2, 3, 5, 7 and 9.
Interestingly, only a few instructions for using the account were found on the network, but all of them are copy-paste from the book “Amusing Arithmetic” by Ya.I. Perelman , which says that the division into seven is too difficult, so according to the seven nothing was found . Of the familiar elderly people who found the time of active use of the account, not one knows how to do this, it was they who did not use the accounts. ')
Maybe someone knows (or can ask someone who knows) how to divide by seven?