Sandy can do a job in 15 days and Molly can do the same job in 30 days. If they work together, in how many days will they complete the job? Choices:
A) 8  B) 9  C) 10  D) 11  E) 12
Sandy can do 1/15 of the job each day.   Molly can do 1/30 of the job each day.   The combined rate is 1/15 + 1/30 = 1/10 of the job each day.   The job will take 10 days.   The answer is 10.
<result>C</result>
Sandy can do a job in 15 days and Molly can do the same job in 30 days. If they work together, in how many days will they complete the job?