Probability Help!

[Austin1]

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How do you do this problem: There are 7 green marbles and 3 red marbles in a bag. What is the probability that you would pick two red marbles in a row?

 

[cusp of carabelli]

it depends on whether you put the first mable you picked back in the bag and picked again....
if you do put it back then it would be (3/10) (3/10)=9/100
(3 red marbles out of a total of 10 mables to choose from)

 

[Viraj]

cusp is right, but if u don't put the marbles back then it will be 3/10* 2/9= 1/15

 

[BingDent]

cusp and viraj is both right.
But if you were to pick two red marbles in a single pick,
the probability of that happening is:

(2!*8!)/10! = 0.022 = 2.2%

Just in case they ask this question.

 

[BingDent]

Oh shoot! made a mistake!
Probability of choosing 2 red in a single pick should be the same as pick 2 red in a row!

P(2 Red) =

(3) (10)
(2)/(2) = 3/45 = 1/15

The reason it's the same as the previous one is because withdrawing two at the same time is same as withdrawing one by one twice without replacement!