Sandy can do a job in 21 days and Molly can do the same job in 42 days. If they work together, in how many days will they complete the job?  Answers.
A) 8  B) 10  C) 12  D) 14  E) 16
Sandy can do 1/21 of the job each day.   Molly can do 1/42 of the job each day.   The combined rate is 1/21 + 1/42 = 1/14 of the job each day.   The job will take 14 days.   The answer is 14.
<result>D</result>
Sandy can do a job in 21 days and Molly can do the same job in 42 days. If they work together, in how many days will they complete the job?