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Jack finished 1/2 of work in 4 hours at his rate (1/8). Now, 1/2 of work is left when Tom joins in. If Tom and Jack had same rate of 1/8 (i.e. they each took 8 hrs to finish the task), then they together would have finished the remaining task in 2 hours, and Jack would have worked a total of 6 hours. But, we can see that Tom's rate is less than Jacks. Therefore, they must have taken more than 2 hours to finish the remaining task. This means that Jack must have worked for more than 6 hours total. By now, options A, B, and C are cancelled out. Now, option D says that Jack worked a little above 6 hours (6 hrs meant that their rates were same) and option E says that Jack worked almost 8 hours. A closer look at their rates will suggest that if option E were correct, then Tom's rate would have been almost half of Jack's rate, which is not the case. Hence, option D is correct.