A group of five clerks has 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 of2 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