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.

Well, although it's a good idea of a problem, you've messed one thing up. "0" is not a natural number, even though you don't use it to solve the problem. But the answer to your problem is 69 useful codes.

Posted by rachael on 2005-03-18 02:47:24