I once read on Habre one convenient way, but apparently forgotten it already...

About suchit:

let's say we have 101010

the first three digits(or bit right) is a 5, is simply to remember

0 is just multiplication by two, for a total of 10

1 — multiply by 2 and add unit, for a total of 21

0 — multiply by 2, total epic 42.

\r

Like everything is fine, but when I wrote the random number is more(this was 91), switch right it could not have...

Maybe someone knows about this method?