28
27
24
24 of them.
24
24 different numbers.
Well, honey, single digit factors of a number are just the numbers that can divide into that number without leaving a remainder. So, for example, the single digit factors of 12 are 1, 2, 3, 4, and 6. It's like finding out who's invited to the party before you start breaking out the cake.
27
24 if you don't repeat any numbers.24
24
24 of them.
24
24 different numbers.
24
27
24 = 4*3*2*1
If you want 4-digit numbers, there are 24 of them.
There are 9 1-digit numbers and 16-2 digit numbers. So a 5 digit combination is obtained as:Five 1-digit numbers and no 2-digit numbers: 126 combinationsThree 1-digit numbers and one 2-digit number: 1344 combinationsOne 1-digit numbers and two 2-digit numbers: 1080 combinationsThat makes a total of 2550 combinations. This scheme does not differentiate between {13, 24, 5} and {1, 2, 3, 4, 5}. Adjusting for that would complicate the calculation considerably and reduce the number of combinations.