Clarification needed: in a box of 10 electrical parts, 2 are defective. (a) If you choose one part at random from the box, what’s the probability that it’s not defective?
The answer is 8/10= 4/5. But why can’t it be 1/8?

Author

Yuilyn Chang

Posted Jun 5, 2018

1 Explanation

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1

Cydney Seigerman, Magoosh Tutor

Hi Yuilyn!

In this scenario, we are selecting one electrical part out of the 10 total that are in the box. The prompt tells us that 2 of the parts of defective. That means that 10 - 2 = 8 parts are not defective. So, the question is asking us how likely is it that we will select one of those 8 parts out of the 10 total parts. For that reason, the probability is 8/10 = 4/5. It's important to remember that the denominator is the total number of options (in this case, all of the parts in the box, which is 10). And in one trial, there are 8 possible not defective parts that we may select.

On the other hand, if there was one unique part that we were asked to select out of a total of 8 parts, we would have a probability of 1/8 of selecting that one part.

## 1 Explanation