Question

how many prime numbers from 11 to 100 are there, whose digits when interchanged give a prime number?

Answer 1

11, 13, 17, 31, 37, 71, 73, 79 and 97.

Answer 2

13, interchanged gives 31 which is a prime.
17 interchanged gives 71 which is a prime.
19 interchanged gives 91 is a prime.
37 interchanged gives 73 which is a prime.
79, interchanged gives 97 is a prime.

So the required answer is 13,17,19,37,31,71,91,73,79,97