6 letters are to be posted in three letter boxes. The number of ways of posting the letter when no letter box remains empty is

Explanation

`l_1 = x; l_2 = y; l_3 = z`

`x+y+z=6`

`x>0, y>0, z>0`

for series

`[1,1,1] -> (6!)/(4!2!!2!)= 15`

`[1,2,3] -> (6!)/(2!3!) = 60`

`[2,2,2] -> (6!)/(2!2!2!3!) = 15`

total ways = `60 + 15 + 15) xx 3!`

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