total 6-
18,19,81,89,91,98
There are 21 two-digit prime numbers.
9
There are five such numbers.
7 two-digit numbers.
20 of them, if repetition is not allowed.
There are 2941 4-digit numbers such no two of its digits differ by 1.
I assume you mean how many 4-digit numbers can be made from a set such as {A,A, B, C} where A, B and C are single digits. There are 12 such numbers.
There are 45 of them.
36
3
90
There are 90 two-digit numbers... starting with 10 and ending with 99.