On any given day, there is an 80% chance that an absent minded professor does research and a 20% chance that he cleans his office. He never does both on the same day. When he does research he always forgets his keys when he leaves his office, but when he cleans his office he only forgets his keys half the time. Last Tuesday, the professor forgot his keys when he left his office. What is the probability he did research on that day?