Source: Official Guide Revised GRE 2nd Ed. Part 9; Section 5; #9

6

# In a quality-control test, 50 boxes

In a quality-control test, 50 boxes—each containing 30 machine parts—were examined for defective parts. The number of defective parts was recorded for each box, and the average (arithmetic mean) of the 50 recorded numbers of defective parts per box was 1.12. Only one error was made in recording the 50 numbers: “1” defective part in a certain box was incorrectly recorded as 10. Quantity A : The actual average number of defective parts per box Quantity B : 0.94 Quantity A is greater., Quantity B is greater., The two quantities are equal., The relationship cannot be determined from the information given.

### 3 Explanations

1

Luz Ramirez

i still don't understand why we subtracted by 9? I understand how we can get 56 as the sum for the erroneous average calculated but don't understand why we just don't but subtract 56-1 and divide it by 50 to get us the actual average.

Jul 9, 2019 • Comment

Okay, so originally, there were 56 recorded defective parts. However, one box was marked as having 10 defective parts in it, when in reality it only had 1 defective part in it. That means there were 9 extra defective parts counted. To find the real sum, then, we have to take away those 9 parts that were labeled as defective but are actually fine. The actual sum of defective parts, then, is 56 - 9 = 47.

1

Shubhangi Kukreti

Why are we dividing with 50 and not 1500 when finding out the new average?

Aug 23, 2018 • Comment

David Recine, Magoosh Tutor

We divide by 50 because there are 50 boxes, and we're looking at the average number of defective pieces PER BOX. 1500 is the total number of parts, regardless of how many boxes there are, so (number of parts)/1500 would actually be the percentage of parts that are broken out of all parts in all boxes, total.

3 Chris Lele, Magoosh Tutor

Dec 8, 2012 • Comment