perseverant wrote:
In a factory that produces computer circuit boards, 4.5 percent of all boards produced are found to be defective and are repaired before being sold, but 10 percent of all defective boards are sold without being repaired. What percentage of boards produced in the factory are defective?
A. 4.5%
B. 5.0%
C. 6.0%
D. 10.0%
E. 14.5%
Another approach is to use the
Double Matrix Method.
This technique can be used for questions featuring a population in which each member has two characteristics associated with it (aka overlapping sets questions).
Here, we have a population of circuit boards, and the two characteristics are:
- defective or not defective
- repaired or not repaired
Since we're looking for a certain
percentage, let's make things easy on ourselves and examine a population of 100 circuit boards.
We're told that
4.5 percent of all boards produced are found to be defective AND are repaired 4.5% of 100 = 4.5
So, we'll place
4.5 in the top-left box, since it represents circuit boards that are defective and have been repaired.
Next, we're told that
10 percent of all defective boards are sold without being repairedSince we aren't told how many defective boards there are, let's let
x = the total number of defective boards among the 100 boards
If there are
x defective boards, then
0.1x represents the number of defective boards that are NOT repaired.
We get the following:
At this point, recognize that the two boxes in the top row must add to
xSo, we can write:
4.5 + 0.1x = xSubtract 0.1x from both sides to get: 4.5 = 0.9x
Divide both sides by 0.9 to get: 5 = x
If x = 5, then we know that there are 5 defective circuit boards out of a population of 100 circuit boards.
5/100 = 5%, so 5% of the circuit boards are defective.
Answer: B
This question type is
VERY COMMON on the GMAT, so be sure to master the technique.
To learn more about the Double Matrix Method, watch this video:
EXTRA PRACTICE QUESTION