Machine A takes 20 hours to complete a certain job and starts that job at 6AM. After four hour of working alone, machine A is joined by machine B and together they complete the job at 1PM. How long would it have taken machine B to complete the job if it had worked alone for the entire job?
Choose one:  A) 9.99  B) 3,615  C) 3.62  D) 4.62  E) 6.14
Let us assume total job = 100 units   A finishes 100 units in 20 hrs(given)   Hence A (working rate)= 5 units /hr   Now Given that A works for 4 hr( so 20 units done) then A and B finish total work in 7 hours. Hence A and B finish 80 units in 3 hours. Of these 3x5=15 units were done by A. Hence B did 65 units in 3 hours.   Hence B(working rate)= 65/3 units /hr   Hence B takes 100x3/65 = 4.62 hours to complete the job.   Answer 4.62.
<result>D</result>
Machine A takes 20 hours to complete a certain job and starts that job at 6AM. After four hour of working alone, machine A is joined by machine B and together they complete the job at 1PM. How long would it have taken machine B to complete the job if it had worked alone for the entire job?