Working alone, A can complete a certain kind of job in 12 hours. A and D, working together at their respective rates, can complete one of these jobs in 4 hours. In how many hours can D, working alone, complete one of these jobs?  Choose the correct choice from the following answers.
A) 1  B) 3  C) 4  D) 6  E) 9
Let total time taken by S to complete the job = S   total time taken by A to complete the job = 12   Work done by A in an hour   1/A = 1/12   Working together A and 6 can complete the job in 4 hours   1/A + 1/6 = 1/4   =&gt;1/6 = 1/4 - 1/12   = 1/4 - 1/12   = 1/6   =&gt; 6 = 6 hours   Answer 6
<result>D</result>
Working alone, A can complete a certain kind of job in 12 hours. A and D, working together at their respective rates, can complete one of these jobs in 4 hours. In how many hours can D, working alone, complete one of these jobs?