There are 4 keys and 4 locks. What is the maximum number of times you need to try

Q: There are 4 keys and 4 locks. What is the maximum number of times you need to try the locks so as to match all 4 keys to their locks?
A. 4
B. 6
C. 10
D. 16

Answer: B

If you try 3 keys on the 1st lock, you will know that the 4th key is a match. So, you only need a maximum of 3 tries. Similarly, the 2nd lock needs a maximum of 2 tries and the 3rd lock needs only 1 try. Thus, 3 + 2 + 1 = 6 tries are needed.