probability question (1 Viewer)

[paper cup]

Braindead, braindead, braindead....I can't believe I can't do this question, it's 2 Unit, and probability, the topic that you will always obtain full marks in unless you read the question wrong. *bites nails*

Help, anyone?

A manufacturer finds that 15% of the articles made are defective. If three articles are taken at random, find the probability that there are more defectives than non defectives.

So P(D>nD) = P(nDDD) + P(DnDD) + P(DDnD) + P(DDD) = (3/20 x 3/20 x 17/20) cubed + 0.00375 = something very different to the given answer of 0.0225 (= 3/20 x 3/20).

What the fuk am I missing?

 

[wogboy]

(3/20 x 3/20 x 17/20) cubed + 0.00375

I believe it should be (3/20 * 3/20 * 17/20)*3 + 0.003375 = 0.06075 instead, but even then it still doesn't match the given answer.

The given answer of 3/20 * 3/20 = 0.0225 is the probability that the first two items picked are defective (A), but this does not match the criteria of the question asked: "Find the probability that there are more defectives than non defectives" (B). What if either one (but not both) of the first two items picked, and the third item picked were defective? (B) is satisfied but not (A). So it could be an error in the question/answer.

 

[silvermoon]

i dont really understand what you mean wogboy - the answer shows the calculations for 2 out of 3 items being defective, which is what the question asked. unless i misunderstood the question (which is really quite likely ) it doesnt matter in this particular question what order the defective ones are picked, so long as there are at least 2.

 

[mizz_smee]

but to find out that probability of the defective ones you have to use all combinations because it doesn't ask for a certain order so therefore can be in any order changing the probability of it happening

 

[silvermoon]

if u do 1-(prob of getting more non-defective) u should get the right answer. id check it myself but...*laughs*...im waaaaay 2 lazy...not to mention thatd be a little 2 geeky for my liking. oh god, what m i doing here? what have u ppl done 2 me? im posting in maths forums...im turning into a maths geek. NOOOOOO! *runs away scared*