The discussion on prediction markets in the chapter on the price system in Tyler Cowen and Alex Tabarrok's Modern Principles of Economics, Second Edition (pp. 123-124) contains the following paragraphs:
Members of HP’s sales team bought and sold shares that paidoff when sales fell within a certain range. A typical security would pay out $\\\$1$, if and only if future sales were, say, between $10,000$ and $15,000$ units. Another might pay off if sales were between $15,000$ and $20,000$ units. The market con- tained $10$ types of securities—a range broad enough to include all the relevant possible sales outcomes.
By examining the prices of all $10$ shares, HP could assign a probability to any combination of outcomes. For example, if the price of the $5,000–10,000$ unit sales security was $10$ cents and the price of the $10,000–15,000$ unit sales security was $20$ cents, this suggests that the probability of selling $5,000 –15,000$ units was $30\%$.
How did the authors arrive at this figure of $30\%$?