Quoting myself from another thread:

Quote Originally Posted by spoonitnow


Tommy picks a marble from the bag. The odds against him picking a blue marble are 5:2 = 2.5:1. The chance of him picking a blue marble are 2/7 = 1/3.5 = 0.286 = 28.6%. In this example, we see that 2.5:1 = 1/3.5.