ISEE Mathematics Achievement Question 315: Answer and Explanation

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Question: 315

15. Hidden Glen Elementary school is collecting donations for a school charity drive. The total number of students in Mr. Greenwood's history class donate an average of y dollars each. The same number of students in Ms. Norris's science class donate an average of z dollars each. In terms of y and z, what is the average amount of donations for each student from both classes?

  • A.
  • B.
  • C. (y + z)
  • D. 2(y + z)

Correct Answer: B

Explanation:

B Since the question involves averages, use the Average Pie . However, save yourself some time by reading carefully! Notice that the classes have an equal number of students donating money. Because of this, simply Plug In values for the averages since it doesn't matter how many actual students are donating money from each class. You only need values for the average of each class, so start there. Those two averages will become the numbers for your total in the next part of the problem. For instance, Plug In $3 as the average, y, for Mr. Greenwood's class, and $5 for z, the average for Ms. Norris's class. Add these two numbers to find the total amount of money donated (3 + 5 = 8), and put 8 in the total spot. There are 2 classes donating money, so the # of items is 2. Find the average by dividing: = 4. The target answer is 4. Now, Plug In 3 for y and 5 for z to find which choice yields 4, the target answer. Eliminate (A) because ≠ 4. Choice (B) works because = = 4. Eliminate (C) because 3 + 5 ≠ 4. Finally, eliminate (D) because 2(3 + 5) = 2(8) = 16, which is way too large. The correct answer is (B).

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