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.

the answer is 62

11
12
13
14
15
18
20

...ect

7 7 7 7 7 7 7 7 6 (6 being that 100 is not valid)

11 is valid due to the fact that 11 is the same upsidown

in conclution 72

Posted by jo on 2005-05-02 06:34:23