A mocha always costs $3 and a cappuccino always costs $2.25. The question stem also indicates that you sold a total of $180 worth today. (Put yourself into story problems; it will help you to keep thinking logically about the story.)
Let's say you sold m mochas and c cappuccinos today. You can write a formula for the revenue earned:
3m + 2.25c = 180
The question asks specifically for the value of m. If you can find m, you can solve. Likewise, if you can find c, then you can plug that into the given equation to find m. So the rephrased question is: What is the value of either m or c?
(1) SUFFICIENT: This sentence allows you to write a second formula: m + 10 = c. You now have two different linear equations and two unknowns, m and c, so it is possible to substitute and solve for each variable. (If you're not sure, start to solve, but stop as soon as you can tell that it's possible to solve.) This statement is sufficient to answer the question.
(2) SUFFICIENT: This statement is more involved. The combined price of all cappuccinos sold is how much money you made selling cappuccinos today. Likewise, the combined price of all mochas sold is how much money you made selling mochas today. Basically, your mocha revenues and your cappuccino revenues today are the same. Since you sold $180 total of these two drinks, you sold $90 in mochas and $90 in cappuccinos.
Since you know the revenue for mochas as well as how much one mocha costs, you can calculate how many mochas you sold. Likewise you could find the number of cappuccinos sold—but don't actually solve for either of these, since that isn't necessary for Data Sufficiency.
You could also think about this information as a second algebraic formula. The combined price of all the cappuccinos sold is 2.25c, or 2.25 times c and the combined price of all the mochas sold is 3m. So 2.25c = 3m. You could use this to substitute into your original equation and solve. Because this is Data Sufficiency, it isn't necessary to actually do that; you just need to know that it's possible to solve.
The correct answer is (D): Each statement alone is sufficient to answer the question.
