It is like a ratio: the specific number of something over the total of everything. Say you have twenty colors, divided into red, purple, yellow, and green. There are five of each. The likelihood of picking red from all twenty is 4:16. It is the same for the rest. The number of something specific compared the to the total. If there is an uneven amount of something like four and six out of ten, the odds are different and there is a better chance that the six will be over the four.
The odds against each other in the picture is 5:15 because they are basically competing to be picked. The second number wouldn't be twenty because you aren't looking for the total but the likelihood something will be picked over another.
It has been a long time since I have done odds so be sure to ask about it.
