103 and 301
236789
18 of them.
There are none.Any number made by using the digits 0 to 9 once is divisible by 9 and so not a prime.There are none.Any number made by using the digits 0 to 9 once is divisible by 9 and so not a prime.There are none.Any number made by using the digits 0 to 9 once is divisible by 9 and so not a prime.There are none.Any number made by using the digits 0 to 9 once is divisible by 9 and so not a prime.
6
014589, if leading 0s are acceptable. Otherwise, 104589.
236789
18 of them.
256
There are none.Any number made by using the digits 0 to 9 once is divisible by 9 and so not a prime.There are none.Any number made by using the digits 0 to 9 once is divisible by 9 and so not a prime.There are none.Any number made by using the digits 0 to 9 once is divisible by 9 and so not a prime.There are none.Any number made by using the digits 0 to 9 once is divisible by 9 and so not a prime.
There are only 6 digits and if they can only be used once then there is no way to string them together to make a number which is 7 (or more) digits in length.
6
501396
396510
3
014589, if leading 0s are acceptable. Otherwise, 104589.
By rearranging the digits the answer is 123.
It is 3567