The answer you are looking for is 15.
To find this, you must first find the total number of people who ate at the restaurant. Multiply 9 and 6 to get 45. The 45 represents the restaurant attendees. Being that it doesn't state the number of plates each person used, we can assume each person used a single plate. So, subtract 45 from 60. Thus getting 15 leftover plates.
I hope this helps!