Since this is DS, and no numerical answer is required, you can also use a theoretical approach:
Stem: We know that 75% of the guests ordered dessert, but this tells us nothing about what percent ordered coffee. It could be anywhere from 0% to 100%.
1) This tells us about the dessert eaters who ordered coffee, but what about those who didn’t order dessert (i.e. the remaining 25%)? Insufficient.
2) This tells us about the same group (coffee & dessert), only as a percentage of coffee drinkers rather than of dessert eaters. We still don’t know how many people had coffee without dessert. Insufficient.
At this point, we know enough to narrow the choices to C & E. While I’m a big fan of the double-set matrix, eliminating choices this way is good exercise, too. After all, we want to focus on what kind of information would be sufficient to solve the problem. Now let’s try combining statements:
1&2) We now know two things about the same group. This is often a good sign that we can solve. In this case, we can find the number of people in this group (60% of 75 = 45), and we know what percent of the coffee drinkers it represents. We can certainly solve this (45=.9C, C=50), but we don’t need to. We know that we have the
ability to calculate the number, and that this number is 90% of the target number. That’s all we need to know. Sufficient.
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Dmitry Farber | Manhattan GMAT Instructor | New York
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