An intellingence agency wants to have codes. For this it uses two digit natural numbers such that the two digits are different. Each of these codes are written on different sheets of paper so as to be used. However, the director of the agency soon realizes that many codes are not uniquely recognisable.
For example, 61 and 19 is one such pair because when the sheet of paper is read upside down, a different number may be read. However, 01 is invalid (no leading zeroes).

How many useful codes are there that the agency can use?
Note: The only digits that make sense when inverted are 0,1,6,8 and 9.

(In reply to No Subject by rachael)

I believe you are right about natural numbers.  That leaves 62 solutions as I figure (71-9=62 (10,20,30...)). 

Now please correct me if I am wrong on these terms. (it has been 14 years since taking any math course).

Natural or counting numbers are whole numbers beginning at 1. (no decimal places)

Real or Whole numbers are the set natural numbers and zero also.(positive).

And integers are the set of whole numbers both positive and negative.

I really do think the author meant real or whole numbers (as i an defining them, correctly or incorrectly)

Edited on April 6, 2005, 4:00 pm

Posted by john on 2005-04-06 15:59:06