Using only sums and differences, and not necessarily all four numbers, 1, 3, 9 and 27 will make all numbers from 0 to 40.
They are prime numbers.
Lots of different combinations can be used, one being: 1x2x3x15 We can see that in any order we multiply these numbers in it will equate to 90.
Only the numbers used in binary code are used twice while all the other numbers are only used once. This numbers are alphabetically arranged ..
Any pair of digits (not including 0), can be used to generate 14 four-digit numbers. If one of the digits is 0, only seven will start with a non-zero digit.
It is a permutation problem (any number can be used repeatedly), it will become 4 x 4 x 4 x 4 = 256 four digit numbers. If each number can only be used once, then the answer is 4 x 3 x 2 x 1 = 24 four digit numbers.
The only numbers used in binary are 0 and 1
There are really only three main counting systems: roman numerals, tallys, and Arabic numbers. Arabic numbers are the simplest to make large number combonations.
Four quantum numbers are used to describe electrons in atoms.
Ten different digits can be used to make 10C4 = 10*9*8*7/(4*3*2*1) = 210 four-digit numbers. Either numbers starting with 0 are permitted or the 10 digits do not contain a 0.