Quandaries and Queries


middle school
Question - Suppose 2 solid color marbles and 2 striped marbles are placed in a box.  All are the same size.  If one marble is randomly drawn from the box and replaced, then a second marble is randomly drawn, what is the probability that the marble drawn both times will be striped? 
I know there is a simple formula for working this out but I can't remember how.


Hi Karyn,

I wouldn't use a formula. I think it is easier to analyze the situation.

The first marble is selected from a box with four marbles, two of which are striped. When you select a marble at random you have two chances in four to select a striped marble. Thus the probability of selcting a striped marble is


When you replace this marble to draw again you have the same situation as for the first draw, two of the four marbles are striped. Thus the probability of selecting a striped marble is again 1/2.

To satisfy the condition you want you need the first ball to be striped AND the second ball to be striped. This is

 1/2  1/21/4.



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