79
This number is a prime.
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Kari Phillips ©2003 
Choose two 2digit numbers of the form n and n+2 from
different decades (for our purpose two 2digit 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
79. For example, 58 and 60 becomes 85  6 = 79. [
Green]
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