Red disks are numbered 1 through 6, the odd numbers are 1, 3, and 5, so there are 3 red disks with odd numbers.
The total number of disks in the box are both red and yellow, so 6 red + 2 yellow = 8 total disks.
The probability of picking an odd numbered red disk would be the number of odd numbered red disks over the total number of disks:
The answer is 3/8