11

This number is a prime.

There's a curious way to multiply a two digit number by the prime number 11. Add the two original digits and place this sum between them. E.g., 35 * 11. (3 + 5 = 8) and we get 385. Note that if the sum of the two digits exceeds 9 place the units digit of this sum between the two numbers, and add 1 to the hundreds digit. [Franco]