Working alone, A can complete a certain kind of job in 6 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 options
A) 8
B) 10
C) 12
D) 14
E) 16
Let total time taken by D to complete the job = D   total time taken by A to complete the job = 6   Work done by A in an hour   1/A = 1/6   Working together A and D can complete the job in 4 hours   1/A + 1/D = 1/4   =&gt;1/D = 1/4 - 1/A   = 1/4 - 1/6   = 1/12   =&gt; D = 12 hours   Answer 12
<result>C</result>
Working alone, A can complete a certain kind of job in 6 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?