A hint to get started
Count signals by their remaining colour multiplicities
Naval signals are made by arranging coloured flags in a vertical line and the flags are read from top to bottom. How many different signals can be made from 1 green, 3 red and 2 blue flags if
(i) all 6 of the flags are used,
(ii) at least 5 of them are used?
CJC · Discussion · Question 7
Same-colour flags are indistinguishable; split five-flag cases by the omitted colour.