There are 21 two-digit prime numbers.
There are 17 of them.
7 two-digit numbers.
There is only one such number.
There are 90 two-digit numbers... starting with 10 and ending with 99.
There are 6804 such numbers.
There are 90 two digit numbers, 10 through 99. There are 18 two digit numbers containing a 9. They are 19, 29, 39, 49, 59, 69, 79, 89, and 90 through 99. Therefore, there are 72 two digit numbers that do not contain a 9.
27: 077,177,277,377,477,577,677,877,977,717,727,737,747,757,767,777,787,797
There are 21 two-digit prime numbers.
There are 17 of them.
There are five such numbers.
7 two-digit numbers.
There is only one such number.
18 positive integers and 36 integers (negative and positive)
There are 2941 4-digit numbers such no two of its digits differ by 1.
36
There are 45 of them.