Answer:
3 attendants 
Explanation:
From 12 p.m. to 3 p.m., that's three hours. 
So if the ticket attendant sells 20 tickets every hour, then you must multiply the number of tickets to the number of hours. 
20 * 3 = 60
The ticket attendant needs 3 attendants from 12 p.m. to 3 p.m.