Probability Question

Jan_de_Bakker

Hi,

A bowl contains two Snickers bars, two Mars bars, and one packet of sunflower seeds. I pick two items out of the bowl at random with replacement. Find the chance that
a) I get the sunflower seeds ... snip

OK so this part a should be the easiest part, I got the other more complex parts right so I don't understand why the answer to this is 9/25 (0.36)

I get 2/5 (0.4)

My reasoning

5 items in total 1 pack of sunflower seeds.

1st run 1/5

2nd run (with replacement) 1/5

its or , so 1/5 + 1/5

?

Is it because Im not including the probability of getting 2 in a row ?

MarinersBlues

I calculate the chance of not getting the sunflower seeds on either attempt and subtract from 1.

(4/5)*(4/5)=16/25

1-(16/25)=9/25

MarinersBlues

Alternatively

Getting them 1st time only
(1/5)*(4/5)=4/25
Getting them 2nd time only
(4/5)*(1/5)=4/25
Getting them both time
(1/5)*(1/5)=1/25

and sum = 9/25

Deise Vu

The answer is 100%. Who can't tell the difference between a packet of Sunflower Seeds and a Mars / Snickers?

Jan_de_Bakker

Thanks, of course so easy when I see it that way ..

I had gotten 9/25 by thinking

25 total outcomes

and these are the 9 im interested in ..

sf/sf
sf/mars1
sf/mars2
mars1/sf
mars2/sf
sf/snick1
sf/snick2
snick1/sf
snick2/sf

But that way is ****e ... thanks

Jan_de_Bakker

Actually, if I do the same without replacement ..

its 8/20

1 - P(not getting sflower)

1 - [4/5 * 3/4]


But if I do :

getting 1st time only 1/5 * 3/4 = 3/20
getting 2nd time 4/5 * 1/4 = 4/20
both times (impossible here)


sum 7/20 ?

spock.

Jan_de_Bakker wrote: »

Actually, if I do the same without replacement ..

its 8/20

1 - P(not getting sflower)

1 - [4/5 * 3/4]


But if I do :

getting 1st time only 1/5 * 3/4 = 3/20
getting 2nd time 4/5 * 1/4 = 4/20
both times (impossible here)


sum 7/20 ?

Getting it the first time only is actually 1/5 * 4/4 = 1/5
If you got it on the first attempt you can't get it again!

This leads to 1/5 + 1/5 = 2/5 = 8/20