59

This number is a prime.

Choose two 2-digit numbers of the form n and n+4 from different decades (for our purpose two 2-digit numbers are from different decades if their leftmost digits are not equal). Now reverse the digits of these two numbers and subtract the smaller from the larger. The result is always 59. Examples: 29 and 33 becomes 92 - 33 = 59, 57 and 61 becomes 75 - 16 = 59. [Green]