If the letters of the word 'mother'are written in all possible order and these words are written out as in dictionary ,find the rank of the word 'mother'.

There are 6 letters, each different
Before MOTHER, words from E , H will come
Words from E = 5! (there are 5 letters to arrange)
Words from H = 5!( there are 5 letters to arrange)

Now words starting by M will come. 
First ME will have = 4! (as 4 letters will be arranged)
Then MH = 4!
Now words with MO
First words starting with MOE... =3!
And words starting with MOH... =3!
the words starting with MOR... =3!

Now, words starting from MOT will come
words starting with MOTE.. = 2!
then after words starting from MOTH will come and first word will MOTHER

so, total no. of words before MOTHER = 5! + 5! + 2* 4! + 3 * 3! 
= 120 +120 + 48 + 18 
= 306 
so, rank of MOTHER = 307