89

This number is a prime.

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Photo credit: Marsha Greer

Choose two 2-digit numbers of the form n = a multiple of ten and n - 1. Now reverse the digits of these two numbers and subtract the smaller from the larger. The result is always 89. For example: 29 and 30 gives 92 - 3 = 89, 89 and 90 gives 98 - 9 = 89. [Green]

89 is a common octane rating for gasoline in the United States. [Green]