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what is the probablity the second mouse will be white?

there are 4 white mice and 6 brown mice in a cage. Molly will take out 2 mice at random. If the first mouse she takes out is brown.
 
a. 2/5
b. 4/9
c.1/2
d.4/5

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Philip P. | Effective and Affordable Math TutorEffective and Affordable Math Tutor
5.0 5.0 (428 lesson ratings) (428)
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After taking out the first mouse (a brown one), there are 9 mice remaining in the cage, 4 white ones and 5 brown ones. The probability, then, that the second mouse will be a white one is computed as follows:
 
P = (Number of White Mice Remaining in the Cage) / (Total Number of Mice Remaining in the Cage)
 
Let me know what you get.
 
 
Kirill Z. | Physics, math tutor with great knowledge and teaching skillsPhysics, math tutor with great knowledge...
4.9 4.9 (174 lesson ratings) (174)
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Notice that the answer will depend on the manner she takes mice out. If she takes them one after another, then Robert's answer is correct. If she takes both together (with two hands, for example) then the probability is:
 
6/10*4/9=4/15

Comments

It is not 4/15. This (4/15) is the probability that the first mouse is brown and the second mouse is white. For this case, it is correctly computed by calculating  4/15 divided by the probability that the first mouse was brown.
 
4/15 divided by 6/10   which is 4/9    the correct answer....