A yardstick's shadow is 4ft. long. A tree's shadow is 20ft. long....

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.
 
The yardstick, its shadow, the distance between the top of the yardstick and the tip of the shadow form a right triangle. Likewise, the tree, its shadow, and the distance between the top of the tree and the tip of the shadow also form a right triangle. These triangles are similar. Since we know that tree's shadow is five times as long as the yardstick's, by similar triangles, the tree will be five times higher than the yardstick. Since a yard is 3 feet, the tree is 3*5 = 15 feet high.

I hope this helps!
 
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