Q.
The number of six lettered words which can be formed from the letters of the word 'HUDHUDBIRD' if each word contains not more than three distinct letters is
HUDHUDBIRD ⇒ (DDD), (HH), (UU), B, I, R
(i) 3A+2OA+1D⇒1×2C1×4C1×3!2!6!=4⋅5!
(ii) 2A+2OA+2OOA=3C3×2!2!2!6!=43⋅5! ∴ Total number of words =(4+43)⋅5!=(419)⋅5!