At a certain restaurant, the ratio of the number of cooks to the number of waiters is 3 to 10. When 12 more waiters are hired, the ratio of the number of cooks to the number of waiters changes to 3 to 13. How many cooks does the restaurant have?
Choose the correct choice from the following.
A) 4 B) 6 C) 9 D) 12 E) 15
Originally there were 3k cooks and 10k waiters.   13k = 10k+12   k=4   There are 12 cooks.   The answer is 12.
<result>D</result>
At a certain restaurant, the ratio of the number of cooks to the number of waiters is 3 to 10. When 12 more waiters are hired, the ratio of the number of cooks to the number of waiters changes to 3 to 13. How many cooks does the restaurant have?