According to Guesstimation 2.0, it would take about 10,000 bullets to cut down a tree with a 20 cm radius. The book doesn’t just announce the result but shows how you might come to this conclusion by computing how much energy it would take to fell the tree and how much energy bullets deliver. After going through a rough calculation, the book reports that Mythbusters actually cut down a tree, a smaller one, with about 2,000 bullets.
Guesstimation 2.0 explores about 100 questions with back-of-the-envelope solutions. (I started to add up the number of problems by looking at the table of contents, but it would be more in line with the spirit of the book and say that since it has on the order to 10 questions and on the order of 10 problems per chapter, it has about 100 questions.)
Another question I found interesting was estimating the amount of fuel needed to transport food across the United States. If you lived in New York and all of your food came from California, it would take about 10 gallons of fuel to bring you your groceries. It may take more fuel to bring your groceries home from the grocery store than it takes to bring the food from around the country to the grocery store.