Question 960264
First draw out the disks and color each one


<img width = 60% src = "http://i150.photobucket.com/albums/s91/jim_thompson5910/disks_zpsovwp3nxb.png">


We are told that "an odd-numbered disk is selected" since it is given to be true. In other words, it is guaranteed to happen or this bit of information is fully known.


So what we do is highlight the disks that have odd numbers on them. I'm going to place them in boxes.


<img width = 60% src = "http://i150.photobucket.com/albums/s91/jim_thompson5910/disks2_zpszu3wpux0.png">

Then erase the ones that weren't placed in boxes. This is to avoid dealing with the disks we know that we haven't selected (remember: we are told or we know 100% that we selected an odd number disk)


<img width = 60% src = "http://i150.photobucket.com/albums/s91/jim_thompson5910/disks3_zpsrneevijo.png">


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It asks: "find the probability of selecting a red disk"


There are 7 disks left over in the boxes that are odd. There are 3 red disks that are left over.


So the probability of picking a red disk from this leftover group is 3/7


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Final Answer: <font size=5 color="red">3/7</font>


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