Unlike many of the questions in the quant section of the GMAT, which can be tricky and getting the right answer to which can require seeing a particular angle, some of the quant oriented questions in the IR section are relatively straightforward, and one of the biggest aspects of what it takes to get right answers to them is merely being very careful. For instance, some of the questions include graphical information, and to get them right you typically have to be pretty careful to make sure you are getting the right values when you use the graphs.
This question is one for which getting it right really just involves carefully following directions. There's no trick to it really. You just have to make sure that you are not missing anything.
Really I think the best angle for you here is to carefully work through the flow chart until you get to the right answer. The skills you would develop in doing that would be more valuable than any particular explanation someone could provide.