"Annie and Evelyn observe that, on average, it takes approximately 10 days for a small glass of water to completely evaporate when left outdoors. What would they estimate for the average annual precipitation on Earth?" (Answer in depth of precipitation) Here's my though process but I have a feeling it is a bit off, if you could help me out it would be greatly appreciated! Thanks! Ok, so first I assumed that a small glass holds 250mL of water and is 10cm tall, and that all water that evaporates in a year falls in that same year, so the quantity evaporated = quantity precipitated. So the Volume of the glass = 250mL = 250cm^3 Thus 250cm^3 = Surface area of the glass x 10(cm) Thus the surface area of the glass is 25cm^2 So it takes 10 days for 250mL of water to evaporate from an area of 25cm^2, and thus in 1 day, 25mL of water will evaporate from that surface area. Then i found the surface area of the earth, and took 71% of that (the part of earch covered in water) and found the annual evaporation of water. Then I spread that out over the entire earth's surface area to find the average annual precipitation in depth.