Is the answer in the back of the book incorrect? It says "out of the 80 nondefective switches, 1/4 are accidentally rejected, which means that 20 are accidentally rejected."

But if I look back at the question, it says that 1/20 of the nondefective switches are rejected by mistake. So I got "B" as the correct answer, but the official answer in the book is "C".

It does appear to be an error...

Here's the solution and process to solve:

We can use the same chart they did, but fix the numbers to reflect the wording in the problem in the text:

Rejected Sold Total
Nondefective 20 60 80
Defective 16 4 20
Total 36 64 100

Thus, we have 4 defective switches sold out of 64 total, which is 4/64 or 6.25% (C)

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So just to confirm, if we went with what was originally asked (1/20 of the non-defective switches are rejected by mistake), then:

Rejected Sold Total
Nondefective 4 76 80
Defective 16 4 20
Total 20 80 100

And therefore the answer SHOULD be 4/80, which is (B) 5%, correct?

yes. that is correct.

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