Mary, Bernie, and Rose can complete a job, all working together, in 4 hours. Mary and Bernie, working together at their respective rates, can complete the same job in 5 hours. How long would it take Rose, working alone, to complete the entire job?	Choose the correct choice from the following options:
A) 8 hours	B) 10 hours	C) 12 hours	D) 20 hours	E) 21 hours
If Mary, Bernie, and Rosecan complete a job in 4 hours, they can complete 1/4 of the job in an hour. Furthermore, if Mary and Bernie can complete the same job in 5 hours, they can do 1/5 of the entire job in an hour.   If the three of them can do 1/4 of a job in an hour, and without Colleen the other two can do 1/5 of a job in an hour, then the amount of the job Colleen can do in an hour is the difference of these results:   1/4 – 1/5 = 5/20 – 4/20 = 1/20.   Since Colleen can do 1/20 of the job in an hour, it will take her 20 hours to do the entire job by herself.   The correct answer is choice (20 hours).
<result>D</result>
Mary, Bernie, and Rose can complete a job, all working together, in 4 hours. Mary and Bernie, working together at their respective rates, can complete the same job in 5 hours. How long would it take Rose, working alone, to complete the entire job?