How many different 3 digit numbers can be found using 3 of the digits 6, 7, 8, 9 only once?
How many lots of 3 digits can you have?
678
679
689
789
4 lots That's it.
But, each 3-digit number can be arranged in 6 different ways, e.g.
678
687
768
786
876
867
So, the number of 3 digit numbers, using 3 of the digits 6, 7, 8, 9 once only, is : 4*6 = 24