What's your hurry ? The night is young.
The first digit can only be 5 ... 1 possibility.
The 2nd digit can be any one of 10. For each of those . . .
The 3rd digit can be any one of 10. For each of those . . .
The 4th digit can be any one of 10. For each of those . . .
The 5th digit can be any one of 10. For each of those . . .
The 6th digit can be any one of 10. For each of those . . .
The 7th digit can be any one of 5. (1, 3, 5, 7, or 9).
Total possibilities: (1 x 10 x 10 x 10 x 10 x 10 x 5) = 5 x 10⁵ = 500,000
I have no idea how many of them exist, but that's the quantity that could exist.
