s1,s2,s3.... s10 are the speakers in a conference. If s1  addresses only after s2 , then the number of ways the speakers addresses is

The number of ways the speakers addresses is =10!2!(∵the  order  of  s1&s2  is   not considered ) (The number of permutations of ‘n’ different things where order of ‘r’ things is not to be considered is n!r!)