a group of investors wants to develop a chain of fast-food restaruants. In determining potential costs for each facility, they must consider, among other expenses, the average monthly electric bill. They decide to sample some fast food restaruants currently operating to estimate the monthly cost of electricity. They want to be 90% confident of their results and want the error of the interval estimate to be more than $100. They estimate that such bills range from $600 to $2500. How large a sample should they take?