suppose there are 6 people standing in the line. In which 3 people
  with  10 rupee note and 3 people with  5 rupee coin
  
 let a denote  people giving 5 rupee coin 
 
 let b denote  people giving 10 rupee note

 possible number of ways without stopping that is when cashier has change all the time
 
  a  a  a  b  b  b
  a  a  b  a  b  b
  a  a  b  b  a  b
  a  b  a  b  a  b
  a  b  a  a  b  b

 now you can see there are (6-1) combinations with 3! combination for
  each 5 rupee coin and 10 rupee note
 n(E)= total number of ways =(6-1)*3!3!=180
 n(s)=exhaustive case as 6 people can stand in any way =6!=720
 
probability= n(E)/n(s)= 1/4
 
 1/4 is nothing but 1/(1 + (6/2) )