Answer
The question you ask can be understood in different ways. I give three.
If you mean showing a sequence of numbers and then being asked to repeat that sequence, the answer is seven (but some people only go up to five and others back to nine, which is why they sometimes say “seven, plus minus two”). It is important that you do not have time to look for a pattern, so it is just numbers one after the other. Moreover, I have to say right away that how you do it can be of great importance. For example, it makes a difference whether you can only look at the numbers and then repeat or you can also read them aloud and then repeat. If you are allowed to read aloud, it turns out that the language plays a role (English, Dutch, Chinese, …), which is surprising.
If you get enough time, you can try to find tools. If a friend’s phone number were 123456789, everyone would remember this nine-digit sequence without any problem. On the other hand, 784243712, also nine digits, will be quite a bit more difficult.
Finally, even if there were no easy pattern to be found, if you repeat a series of numbers enough, it will also stick in your memory, but that is another situation. That is training your memory through practice.
Answered by
prof.dr. Jean Paul VAN BENDEGEM
logic, philosophy of mathematics and philosophy of science
Pleinlaan 2 1050 Ixelles
http://www.vub.ac.be/
.