103 and 301
236789
18 of them.
The largest even number less than 400,000,000 that can be formed using the digits 1 through 9 only once is 398,765,412. This number uses each of the digits from 1 to 9 exactly once and ends with the digit 2, making it even. Other combinations either exceed 400,000,000 or do not use all the digits exactly once.
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
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
3
014589, if leading 0s are acceptable. Otherwise, 104589.
396510
501396
By rearranging the digits the answer is 123.
It is 3567