A group of five clerks have been assigned to insert 24,000 letters into envelopes .the clerk 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 Take off the assignment, the number of hours required for the remaining clerks to complete this assignment is :