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 Solution? by Timothy Bard)

Timothy Bard wrote:

There are 90 two-digit numbers with no leading zeros (10 - 99).  Out of those, 12 can be flipped to produce a different number:
16 61 81 91
18 68 86 96
19 69 89 98

69 and 96 are uniquely identifiable even when turned upside down.  I think that should bring your total back up to 71.

Posted by Erik O. on 2005-03-10 14:14:54