Quandaries
and Queries
Karyn
middle school
student
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
2/4 = 1/2
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
1/2 = 1/4.
Penny