A group of five clerks have been assigned to insert 24,000 letters into envelopes. The clerks perform this work at the following rates of speed: Clerk A, 1100 letters an hour, Clerk B, 1450 letters an hour; Clerk C, 1200 letters an hour, Clerk D, 1300 letters an hour; Clerk E, 1250 letters an hour. At the end of two hours of work, Clerks C and D are assigned to another task. From the time that Clerks C and D were Taken off the assignment, the number of hours required for the remaining clerks to complete this assignment is: