Since the yardstick (3 ft) casts a shadow 4 ft long, and the tree casts a shadow 20 ft long, you can set up a little ratio of 3:4 = x:20. When you cross multiply the numbers, you get (3 times 20) divided by 4, which is 60 divided by 4. 60/4 = 15. So your tree is 15 ft tall.
