A company operates a zoo in a city. The company adjusts its accounts at the end of each month. Thr following account reflects balance AFTER adjusting entries were prepared on April 30th: "Unearned Ticket Revenue 400 $". The unearned ticket revenue represents tickets sold for future zoo visits. The tickets were sold at 4 $ each on April 1st. During April, twenty of the tickets were used by customers. What is the number of tickets sold on April 1st?