The number of selections of four letters from the letters of the word ASSASSINATION is 

ASSASSINATION contains 4S, 3A, 2I, 2N, and one each of T and O. Four letters can be selected in following ways 1)All four identical in 1 ways 

2) Three identical and one different letter (e.g. AAAN) in${ }^2{\mathrm{C}}_1\times {}^5{\mathrm{C}}_1=10$  ways. 

3) Two identical of one kind and two identical of another kind (eg. SSNN) in ${ }^4{\mathrm{C}}_2=6$   ways.

 4)Two identical and other two different (e.g. IINT) in  ${ }^4{\mathrm{C}}_1\times {}^5{\mathrm{C}}_2=40$ ways. 

5.)All four different letters in  ${ }^6{\mathrm{C}}_4=15$ ways.  

Total number of ways  $=1+10+6+40+15=72.$