Quick Probability question (1 Viewer)

[kloudsurfer]

Hi,
I could someone please help me with this question? Its pretty simple, but i keep getting the wrong answer.

"A box contains disks, each with a number on it. The numbers are 2, 2, 3, 3, 3, 4, 5. Daniel takes one disk out of the box at random and then, without replacing the first disk, he selects a second.

What is the probability that the second disk selected is a two?"

So heres my working out:
P = 2/7 x 1/6 + 5/7 x 2/6
= 2/7
My logic is that the probability of selecting a 2 then a 2 is 2/7 x 1/6, and the probability of selecting any other number then a 2 id 5/7 x 2/6

However, the answer in the book says

P = 1/7 x 1/6 + 5/7 x 2/6
= 11/42

What have i done wrong? Is the book wrong (it has been known to be)

Thanks.

 

[watatank]

i'd go with your answer.

 

[jannny]

yep I agree 2/7

[ 1/7 x 1/6 ] + [ 1/7 x 1/6 ] + 5[ 1/7 x 2/6 ] = 12 /42 = 2/7

 

[ssglain]

I agree with your answer, kloudsurfer.

 

[kloudsurfer]

Yay.

So once again, my textbook is wrong!

It seems to have a lot of wrong answers. Its the Leading edge study guide thing.

Thanks everyone.