Working alone, A can complete a certain kind of job in 10 hours. A and D, working together at their respective rates, can complete one of these jobs in 5 hours. In how many hours can D, working alone, complete one of these jobs? Pick one:  A) 9  B) 10  C) 11  D) 12  E) 13
Let total time taken by D to complete the job = D   total time taken by A to complete the job = 10   Work done by A in an hour   1/A = 1/10   Working together A and D can complete the job in 5 hours   1/A + 1/D = 1/5   =&gt;1/D = 1/5 - 1/A   = 1/5 - 1/10   = 1/10   =&gt; D = 10 hours   Answer 10
<result>B</result>
Working alone, A can complete a certain kind of job in 10 hours. A and D, working together at their respective rates, can complete one of these jobs in 5 hours. In how many hours can D, working alone, complete one of these jobs?