Chapter 12 - Data Analysis and Probability - 12-7 Theoretical and Experimental Probability - Practice and Problem-Solving Exercises - Page 773: 38

.5

We know that the sum of the odds in favor of event a and against event a should be 100 percent. We also know that the odds in favor of event a are equal to the odds against event a. Since the sum must be 100%, this will only be true when event a will happen 50% of the time. Thus, the probability is: 1/2 = .5