A letter lock consists of two rings each marked with four different letters. The number of distinct unsuccessful attempts to open the lock is at the most ? Choose the correct choice from the following answers:
A) 17
B) 19
C) 15
D) 168
E) 167
Since each ring consists of four different letters, the total number of attempts possible with the two rings is   = 4*4= 
<gadget id="calculator">4*4</gadget>
<output>16</output>
16. Of these attempts, one of them is a successful attempt.   Maximum number of unsuccessful attempts= 16 - 1= 
<gadget id="calculator">16 - 1</gadget>
<output>15</output>
15.   Answer: 15
<result>C</result>
A letter lock consists of two rings each marked with four different letters. The number of distinct unsuccessful attempts to open the lock is at the most ?