Ambitious Danish statistician and environmental maverick Bjorn Lomborg recently put the question to eight of the world's sharpest minds in economics, including three Nobel Prize winners. Based on a list of global challenges identified by the UN, Lomborg instructed them to name the world's top 10 worst problems, then rank more than 30 proposed solutions to those problems in terms of spending priority. Never mind politics and cultural differences, he said. Instead, use cost-benefit analysis: Crunch the numbers, and weigh benefits against costs to determine how to get the most bang for a buck.