An accrediting agency set up a proficiency test for a lab in which 20 percent of the samples were injected with a contaminant. If the lab correctly labeled 80 percent of the contaminated samples but incorrectly labeled 5 percent of the uncontaminated samples, then what percent of the samples labeled by the lab as uncontaminated were actually contaminated?
(A) 4%
(B) 5%
(C) 10%
(D) 20%
(E) 25%
(A) 4%
(C) 10%
A.
B
(B)
B…can u please tell if the answer is correct
A
take gmat team, please provide the answer.
A
A
A
B
A
(B) 5%
Let’s take absolute numbers.
Total number of samples = 100
contaminated samples = 20
Therefore, UNcontaminated samples = 80
“lab correctly labeled 80 percent of the contaminated samples”
=>
16 out of 20 were ‘correctly identified’ by the lab as contaminated.
lab missed 4 of them
“Incorrectly labeled 5 percent of the UNcontaminated samples”
=>
5% of 80 i.e. 4 were UNcontaminated but identified as contaminated by the lab.
Therefore, the lab labeled = 16 + 4 =20 samples as contaminated.
Therefore, it labeled 100 – 20 = 80 as UNcontaminated
“what percent of the samples labeled by the lab as uncontaminated were actually contaminated?”
we need -> 100 * (Number of Misses that were contaminated) / (Number labeled UNcontaminated)
= 100*(4)/(80) = 5%
Ans is B
Sorry about the boldface.
TakeGMAT, can I somehow edit/remove my previous comment?
Hi magnus1,
Dont worry, we rectified it.
Thanks
The explanation for B seems plausible. I am afraid I only guessed at A.
B