Table schema:
CREATE TABLE table_18118221_1 (annual_interchanges__millions__2011_12 VARCHAR, annual_entry_exit__millions__2011_12 VARCHAR)

Question: How many annual interchanges in the millions occurred in 2011-12 when the number of annual entry/exits was 36.609 million? 
SQL: SELECT annual_interchanges__millions__2011_12 FROM table_18118221_1 WHERE annual_entry_exit__millions__2011_12 = "36.609"