After the first person buys a ticket the clerk has 1 25 bill
After the second person buys a ticket the clerk has 2 25 bills
After the thirs person buys a ticket the clerk gets one 50 bill, gives back a 25 bill and ends up with 1 25 and 1 50 bill
After the fourth person buys a ticket he gets a 100 and gives back the 50 and the 25 he had, so now the clerk has 1 100 bill