A town council of 7 members contains a steering
committee of size 3. New ideas for legislation go
first to the steering committee and then on to the
council as a whole if at least 2 of the 3 committee
members approve the legislation. Once at the
full council, the legislation requires a majority vote (of at least 4) to pass. Consider a new piece of
legislation, and suppose that each town council
member will approve it, independently, with probability
p. What is the probability that a given steering
committee member’s vote is decisive in the
sense that if that person’s vote were reversed,
then the final fate of the legislation would be


There are some answers on the web for this book problem which are incorrect.) To set up the
problem: note there are 7 voters. Set up voters 1-3 on the steering committee, and voters 4-7 as not
on the steering committee. Focus on voter 1. Thus there are 2^7 = 128 possible outcomes. A key
question obviously is: For how many of these 128 outcomes is voter 1’s vote decisive? To make sure
you understand the problem, it’s best to answer this question for a few example outcomes, before
you start setting up the computation. Would your answer be different if you consider voter 2?


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