1) Ways of choosing 2 marbles = 10C2 = 45
Ways of choosing two red marbles = 3C2 = 3
Probability = 3/45 = 1/15

2) Ways of Choosing 5 marbles = 10C5
ways of choosing 1 red 1 green and 3 blue = 3C1*2C1*5C3

Probabilty = 3C1*2C1*5C3/10C5 = 5/21

3) Probabiliy that first marble is green = 2/10
Probability that second marble is red = 3/9
Total = 2/10 * 3/9 = 1/15

4) Probabiliy that first marble is green = 2/10
Probability that second marble is red = 3/9
Probability that third marble is blue = 5/8
Total = 2/10 * 3/9 * 5/8 = 1/24

same here, I also get confused about when to use which method. I am getting same answer as jay for 1,3, and 4 but not 4. A detailed explanation would be much appreciated.


Is it wrong to use this method for 2?

3/10*2/9*5/8*4/7*3/6=360/30240

Thanks

Note the difference between the following:

When the question says 1 red and 1 green marble, the order is not important. This includes the two possibilities RG and GR, meaning the two chioces where first one is Red, second one is Green and first one is Green, second one is Red.

The problems where order is not important can actually be solved in one of the two ways (Combinations or Multiplications) as explained below:

for example, take question 1 above, what is the probability of getting 2 Red marbles:

problem 1:

Method I:

Total # of outcomes = Total # of different ways 2 marbles can be chosen from 10 marbles = 10C2 = (10*9)/(1*2) = 45

Total # of ways 2 Red marbles can be chosen from 3 red marbles = 3C2 = 3

The probabilty of getting 2 red marbles = 3/45

Method II:

To get 2 red marbles, the marbles that are pulled out from the bag should come out in the order RR (any other order is not permitted).

Hence, the problem boils down to two events where

event 1 - picking 1 red marble out of 10 marbles in which 3 are red
event 2 - picking 1 red marble out of 9 marbles in which 2 are red (note that event two will have only a total of 9 marbles; out of which ONLY 2 are red having already pulled a red one out in event 1).

So, the probability of event 1 = 3/10
probability of event 2 = 2/9

probability of the combined event (1 & 2) = (3/10) * (2/9) = 3/45

Problem 2:

Method I:

choosing 1 red, 1 green and 3 blue out of 10 marbles - 3 red, 2 green and 5 blue marbles

So, again the order is not important.

total # of outcomes = 10C5 = (10*9*8*7*6)/(1*2*3*4*5) = 252

# of selections of 1 red out 3 red marbles = 3C1 = 3
# of selections of 1 green out of 2 green = 2C1 = 2
# of selections of 3 blue out of 5 blue = 5C3 = (5*4)/(1*2) = 10

So, the combined probability = (3*2*10)/(252) = 5/21

Method II:

Note that 1 red, 1 green and 3 blue marbles can come out in any order.

For example, selecting RGBBB, GRBBB, BBRGB, BBRBG, ... etc will all satisfy the condition that you will have 1 red, 1 green, 3 blue marbles at the end.

So, if you want to apply this method to solve the problem, you would have to add the individual probabilities p(RGBBB) + p(GRBBB)+p(BBRGB)+...(all these combinations that would result in 1 red, 1 green and 3blue marbles).

Even though you can take this approach to solve the problem, you notice that the problem has become cumbersome to solve this way.

This is given to show that the problem can be solved either way.

problem 3:

Choosing 2 marbles such that the first one is green and the second is red.

Method I:

There are two events - first one which should yield green and second one which should yield red. You are looking for GR.

Note that getting a Red and then a Green does not satisfy this condition.

Selecting a red and a green includes both these choices "GR" and "RG".

So, the probability of a "GR" is - 2C1/10C1 * 2C1/9C1 = (2*2)/(10*9) = 2/45.

Method II:

In this case, the second method virtually becomes same as method I.

So, the probability = 2/10 * 2/9 = 2/45.

Problem 4

Choosing 3 marbles such that the first one is green, the second is red and the third is blue.

Method I: probability = 2C1/10C1 * 2C1/9C1 * 5C1/8C1 = 2*2*5/(10*9*8) = 1/36

Method II: virtually same = 2/10* 2/9 * 5/8 = 1/36

-mathguru
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http://www.askmathguru.com

Just a note. You should clearly state (or so that it can be inferred) if the selection of marbles is WITH or WITHOUT replacement. The calculations are totally different for these 2 cases