Choice B is correct. From the description given,  is the portion of the job that the two printers, working together, can complete in one hour, and each term in the sum on the left side is the part of this  of the job that one of the printers contributes. Since one of the printers is twice as fast as the other, describes the portion of the job that the faster printer is able to complete in one hour and describes the portion of the job that the slower printer is able to complete in one hour.

Choice A is not the correct answer. The student may have not seen that in this context, the rates (that is, the work completed in a fixed time) of the printers can be added to get the combined rate, but the times it takes each printer to complete the job cannot be added to get the time for both printers working together, since the time for printers working together is less than, not greater than, the times for each printer alone. Hence the terms in the sum cannot refer to hours worked. In fact, the time it would take the slower printer to complete the whole job is x hours.

Choice C is not the correct answer. The student may have seen that  is the smaller term in the sum, wrongly concluded that the smaller term must apply to the faster printer, and then assumed the 2 in the numerator of the second term implies the equation describes work completed in 2 hours. In fact, the portion of the job that the faster printer could complete in 2 hours is

Choice D is not the correct answer. The student may have correctly seen that the value on the right side refers to the portion of the job completed, but not seen that in this context, the rates (that is, the work completed in a fixed time) of the printers can be added to get the combined rate, but the times it takes each printer to complete the job cannot be added to get the time for both printers working together. Hence the terms in the sum cannot refer to hours worked. In fact, the time it takes the slower printer to complete  of the job is  hours.