Question 739939
Job (usually 1) = work rate x time.<P>

Machine 1:  1 = r*6 so 1/6 = rate. <P>
Machine 2:  1 = r*5 so 1/5 = rate.<P>
For them working together:  1 = (1/6 + 1/5)* t = (11/30)* t so 30/11 = t = 2 hours 43.6 minutes.