Question

CAN SOMEONE PLZ HELP?!!

Answer 1

Looks like a binomial situation.
Are you familiar with the binomial formula?

Answer 2

Not A clue.

Answer 3

That's ok, this can be done without it because the numbers are small enough to not get too cumbersome.
First identify your probabilites of success and failure.
prob' of success = p=0.3 (has under influence)
prob' of failure = q=0.7 (has not under influence)
The sample size, n=3.

Answer 4

The first question is asking for at least one not under influence.
You can think of that as the complement of the probability that all three have.
i.e. P(at least one didn't) = 1-P(all three did).

Does that make sense?

Answer 5

sorta. I'm just confused on what #'s to use

Answer 6

If you do it the way I suggested, it's pretty quick. There's a long way to do it that gets the same answer, but who wants to do something the long way?

The probability that they all have = 0.3 × 0.3 × 0.3
(multiplication rule for an AND situation).

Answer 7

So if that's the P(all have), then P(at least one hasn't) is that number subtracted from 1.

Answer 8

Just for general interest, the long way looks like this:
(0.7 × 0.3 × 0.3) × 3 + (0.7 × 0.7 × 0.3) × 3 + (0.7 × 0.7 × 0.7) × 1
[I know, who would want to do it that way?]

Answer 9

so i do 1-0.027?

Answer 10

Yep.

Answer 11

so the answer a) is 0.973?

Answer 12

(Here's a good overview of basic probability rules http://stattrek.com/probability/probability-rules.aspx?Tutorial=Stat I recommend looking at other pages on that site as well)

Answer 13

Yes, that would answer question (a). :-)

Answer 14

[Though, it says to round to four decimal places, so you might want to express it as "0.9730"]

Answer 15

ok so part b?

Answer 16

Same kind of reasoning, but you're going to take the complement of P(all have not) = 1-P(at least one has).

Answer 17

So what is the probability that all three have not operated under the influence of sugar?
[While you figure that out, I'm going to go put more sugar in my coffee...]

Answer 18

lol ok

Answer 19

is it 0.7?

Answer 20

0.7 is the probability that one at random has not, but you want to find the probability that three have not.

Answer 21

well then idk.

Answer 22

you got this or no?

Answer 23

Let's talk more about the multiplication rule...

Answer 24

If you want the probability that all three have not operated under the influence, then that's like . .. let's say the three people are A, B, and C . . . for A to not have, that's a 0.7 prob'
for A AND B to not have, that's 0.7×0.7 . . .
See where I'm going with this?

Answer 25

So, once you find the probability that all three have not, then the probability that at least one has = 1 minus that.

Answer 26

oh ok so i do 0.7 x 0.7 x 0.7 & that =0.343.

Answer 27

Right . . .

Answer 28

then 1-0.343=0.657

Answer 29

so 0.657 is the answer?

Answer 30

That should be it.
You should think through it logically and ask if it makes sense (the last step of any problem solving is the 'reality check.')