Find a nine digit number abcdefghi that uses all the digits 1,2,3,...9 exactly once and satisfies:
a is divisible by 1
ab (2 digit number with digits a and b) is divisible by 2
abc (3 digit number with digits a, b and c) is divisible by 3
abcdefghi is divisible by 9.
Solution: Posted by JDGM, Anti, Meghana Kolan, Shwetabh Sameer and Unknown in comments! A more detailed solution provided by me in comments!