Two variables tells you this is a great place to plug in. Let’s pick numbers that make the  math easy. You can try x = 30 and y = 2. So in 2 hours there are 4 periods of 30 minutes each: 12 × 4 = 48. Alex can fold 48 napkins in 2 hours. 48 is your target. Plugging into your answer choices gives you (C) as the only correct answer.

This is a perfect question for PITA. The question asks for the greatest possible number of 20-ounce boxes. Start with (B). If there are twenty-five 20-ounce boxes, then there are twenty-five 8-ounce boxes because a total of 50 boxes was purchased. In this case, the twenty-five 20-ounce boxes weigh 500 ounces, and the twenty-five 8-ounce boxes weigh 200 ounces; the total is 700 ounces. This is too big because the question says the total weight was less than 600. If (B) is too big, (A) must also be too big; eliminate both answers. If you try (C), the total weight is 604 ounces, which is still too big. So the answer must be (D).

Since you are never told what x is, and there is no way to find it, plug in for x. Say that x = 100. 63% of 100 is 63, and ​\( \frac{3}{8} \)​ of 100 is 37.5. The ratio of a to c is ​\( \frac{a}{c} \)​. So, ​\( \frac{63}{37.5} \)​ = 1.68. To save time, you can ballpark the answer, since a > c and (D) is the only choice greater than 1.

Here’s yet another chance to plug in because of the variables in the answer choices. In this case, you have several variables. You should start by plugging in values for x and y, and then work out c. Because x > y > 0, let’s say x = 6 and y = 3. Therefore, c =​\( \frac{1}{6}+\frac{1}{3}=\frac{1}{6}+\frac{2}{6}=\frac{3}{6}=\frac{1}{2} \)​. The question asks for the value of, which is the reciprocal of, or 2. This is your target answer. If you plug x = 6 and y = 3 into all of the answer choices, you’ll find that only (D) equals 2.

When asked for a specific value, try Plugging In the Answers. Label them as gallons of premium and start with the value in (B). If 75 gallons of premium were sold, the station would make 75($2.79) = $209.25 for those sales. A total of 550 gallons was sold, so the station would have sold 550 – 75 = 475 gallons of regular gasoline. The sales for the regular gasoline would be 475($2.39) = $1,135.25. The total sales for both types of gasoline would be $209.25 + $1,135.25 = $1,344.50. That matches the information in the question, so (B) is correct.