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In box A, there are 20 purple and 30 red marbles. In box B,

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CEO
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Joined: 15 Aug 2003
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In box A, there are 20 purple and 30 red marbles. In box B, [#permalink] New post 05 Oct 2003, 01:15
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In box A, there are 20 purple and 30 red marbles. In box B, there are
15 purple and 35 red ones. if a box is first selected randomly, and then
one marble is selected from that box, what is the probability of selecting
a purple one?

Simple enough , but the answer given is 9/20 ...

thanks
praetorian
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 [#permalink] New post 06 Oct 2003, 10:31
I don't see why it would be 9/20

This is how I tried it:

the probability of choosing box a is 1/2, same with box B

So the probability of getting a purple marble, overall, =
the probability of choosing box A and getting purple + probability of choosing box B and getting purple =
(1/2 * 20/50) + (1/2 * 15/50) = 7/20

So I must be doing something wrong, but don't see what? Anybody can help?
Thanks!
CEO
CEO
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Joined: 15 Aug 2003
Posts: 3470
Followers: 60

Kudos [?]: 674 [0], given: 781

 [#permalink] New post 06 Oct 2003, 13:35
toujours28 wrote:
I don't see why it would be 9/20

This is how I tried it:

the probability of choosing box a is 1/2, same with box B

So the probability of getting a purple marble, overall, =
the probability of choosing box A and getting purple + probability of choosing box B and getting purple =
(1/2 * 20/50) + (1/2 * 15/50) = 7/20

So I must be doing something wrong, but don't see what? Anybody can help?
Thanks!


thats what i got....i think the answer is wrong..

thanks
praetorian
  [#permalink] 06 Oct 2003, 13:35
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In box A, there are 20 purple and 30 red marbles. In box B,

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