zak100 said:
Hi,
Thanks for your response.
probability = favorable outcomes/total outcomes
I think the total outcomes is 4 because at a time we are picking 4 marbles.Now the probability that it is removed is 1/4, because out of 4 only 1 would be the yellow.
And the probability that it is not removed is: 4/4 because it is possible that out of 4 none of them is yellow.
Answer is not correct so please guide me.
Zulfi.
What you wrote above is WRONG. If you had a removal event with probability 1/4, the probability of the non-removal event would have to be 3/4 (because two events {removal} and {non-removal} are mutually exclusive and "exhaustive"---- meaning that together they give all the possibilities and so have probabilities that sum to 1).
Anyway, the 1/4 is also wrong.
You need to think more carefully about what you are doing. You should always approach such problems systematically, until you are a lot more experienced. So, the steps you should always follow are:
(1) describe the "sample space" of the experiment---which is the set of all possible outcomes;
(2) assign probabilities to the individual outcomes in the sample space;
(3) obtain the probability of an event of interest by adding together all the individual-outcome probabilities for those outcomes that belong to your event.
For example, when you draw 4 items from 5 differently-colored items you can have a sample-space consisting of all 4-letter strings of the form ABCD, where A is one of the five colors (R,W,G,B,Y), B is another one of the five colors, etc. You may then be interested in the event NY = {no Y} = {no yellow}; this consists of all the 4-letter strings having no letter Y. So, if you assume that all the 4-letter strings are equally likely then you can say that
$$P\{ \text{no yellow}\} = \frac{\text{ number strings not containing Y}}{\text{total number of strings}}\; \hspace{2mm} (1)$$
Have you computed the numerator and denominator of the fraction in (1)?