Are you sure this is written correctly? Any number whose digits add to 9 is divisible by 9. For a 2 digit number then, your choices are 18, 27, 36, 45, 54, 63, 72, and 81. None of these has the property that for the 2 digit number AB that B=2A-1. Solving your question algebraically is possible but you get fractional digits.
