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.
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)
--------------------------------------------------------------------------------------------------------------
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
--------------------------------------------------------------------------------------------------------------
Final Answer: 3/7
------------------------------------------------------------------------------------------------------------------------
If you need more one-on-one help, email me at jim_thompson5910@hotmail.com. You can ask me a few more questions for free, but afterwards, I would charge you ($2 a problem to have steps shown or $1 a problem for answer only).
Alternatively, please consider visiting my website: http://www.freewebs.com/jimthompson5910/home.html and making a donation. Any amount is greatly appreciated as it helps me a lot. This donation is to support free tutoring. Thank you.
Jim
------------------------------------------------------------------------------------------------------------------------