Lets arrange the letters of OUGHT in alphabetical order.
G,H,O,T,U
Number of words starting with G
=4!=24
Number of words starting with H
=4!=24
Number of words starting with O
=4!=24
Number of words starting with TG
3!=6!
Number of words starting with TH
=3!=6
Number of words starting with TOG
=2!=2
Number of words starting with TOH
=2!=2
Next word is TOUGH.
Hence, the serial number of the word TOUGH is
=24+24+24+6+6+2+2+1=89