5040 different 4 digit numbers can be formed with the digits 123456789. This is assuming that no digits are repeated with each combination.
It is 120 if the digits cannot be repeated.
There are 5*4*3 = 60 such numbers.
27 three digit numbers from the digits 3, 5, 7 including repetitions.
There are 5! = 120 such numbers.
27 if the digits can be repeated. 6 if not.
If the digits may not be repeated, there are 6x5x4x3x2x1 = 720 numbers, If digits can be repeated, the answer is 6x6x6x6x6x6 = 46,656 numbers If zero is ong the available digits but is excluded from the first place. 5x6x6x6x6x6=38,880 numbers
48 of them if digits may not be repeated. 100 if they can.