GMAT Club Tests | D01-29

Maybe I don't understand this completely. I understand that T/M = (R+T)/J. If W is the total work done, then both of these equal W/2. Thus, it's clear that T/M=(R+T)/J=W/2.

How can you say that T/M=1/2, when you don't know how much work was done? Which part of this question allows us to say that W=1, thus T/M=(R+T)/J=1/2?? Ie, which part of the prompt restrict the variables such that we know W=1?

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